Selecting an accessory viewfinder is often a lot more difficult than picking a finder with the correct focal length (or the equivalent) when switching between formats.  The following is a short list of items to look at when you are shopping for an accessory finder.

1.  What is the field of view?  The field of view is defined by the aspect ratio and the frameline size.  When the aspect ratio between two formats is identical, you can match framelines fairly easily.  For example, 6x9cm and 35mm are both nominally 2:3 formats.  So you can multiply a 6x9 lens' focal length by 0.4 to get the equivalent for 35mm.  So a 50mm lens is a 20mm in small camera terms, a 65mm is a 28mm, a 90mm is a 35mm, a 100mm is a 40mm, and a 180mm is a 75mm.  Although these are not exact equivalencies, they are close enough.  Most accessory viewfinders do not cover 100% of the field size, which brings us to our next consideration...

2.  Coverage.  As Leica users are painfully aware, a viewfinder (or frame size) is really only good for a given focal length at one distance.  This is due to the fact that field size increases or decreases with distance.  Many accessory viewfinders have coverage expressed in a percentage of field size (e.g., 88%) at a certain distance (2m).  Ostensibly, this was done to prevent disappointment and problems with parallax error.  But the reality is that not only should the framelines be bigger even at 2m, but if you are shooting mostly distant pictures, you also need much bigger framelines This conceivably could mean the difference between using a 25mm and a 28mm viewfinder - or a 35mm instead of a 40mm.  With true "zoom" finders (Nikon, Canon, Tewe, Leica 21-24-28mm), you can just jog the dial a little toward the next biggest focal length.

3.  Parallax correction method.  Some finders have distance dials that compensate for parralax error, which is the mismatch between what a finder sees and what the lens sees.  This is usually achieved in one of three ways. 

• Tilting the entire viewfinder toward the lens.  This is effective; however, it destroys any ability to use the framelines as a vertical/horizontal reference for wideangle work.  Many Canon rangefinder accessory finders use a pin that connects the to RF mechanism of the camera, making correction automatic on the Canons (like the VI-L) but impossible on anything else.
• Moving the framelines up and down (like a Leica M camera does in its main viewfinder). This works but is present on only one viewfinder, the Leica Universal Viewfinder M. 
• Moving the eyepiece, forcing your eye to look at an angle through the viewfinder.

The alternative (often seen) is a dotted line that runs left-to-right across the top of the frame.  This is the quick-and-dirty version, and it does not show where the bottom of the corrected frame would be (sometimes it does, on one variation of the Voigtlander Kontur).

4.  Parallax correction amount.  Here's something that has never been critically examined.  A viewfinder designed for one camera builds in a parallax correction tha takes into account

• The distance between viewing lens and taking lens.
• The direction between the two.

Viewfinders designed for cameras where the accessory shoe sits directly above the taking lens usually only correct parallax up-and-down.  Viewfinders where the accessory shoe is diagonal from the taking lens usually correct in two dimensions.  This is usually not a problem.  What is, however, a problem is that medium-format cameras often have much larger distances between lens axis and accessory viewfinder, meaning that putting an MF viewfinder on a 35mm camera will result in over-compensation at any given correction distance - and putting a 35mm finder on an MF camera will result in under-compensation.

5. Mounting foot.  Putting a new finder on an old camera can be more of a headache than you think.  If you only used cameras made in the mid-1970s or afterward, you would be deluded into thinking that everything used the same ISO 518 flash shoe (what is commonly called a "hot" shoe, though to be "hot," it needs flash contacts).  That shoe, as you probably know, consists of two sheet-metal rails, a spring clip, a hard stop toward the front of the camera, and some number of flash contacts.  

The basic size and shape of accessory shoes on cameras have been constant for almost 100 years (the Konica IIIM had the first instance of a "hot" shoe - and its shoe is capable of firing modern electronic flashes).  But camera makers used radically different approaches in keeping an accessory in place.  The Germans typically used a spring-loaded mechanism under the shoe to push the accessory up into the top rails.  The Japanese were fans of spring clips in the shoes, though some Japanese finders had leaf springs to keep accessories snug (or had flexible prongs cut into the foot that helped provide tension). 

The problem is that you might not get a secure connection and end up dropping/losing the finder.  This is a big problem in attaching newer finders to older shoes that lack spring-loading (such as on the Fuji G690/GL690/GM670 cameras, all of which are designed to accept either Fuji finders with "prongs" to maintain tension or flash units that have their own lock-on mechanisms).  Conversely, it is often difficult to appreciate from pictures on Ebay that the foot on a Kontur or Turnit finder is much longer than on a modern accessory.  This can create issues with ergonomics if the accessory sticks too far out backwards.  Whatever you do, don't force any accessory into any finder.

6. Eye relief. Somewhere along the way, when everyone was so excited about the tiny, plastic-bodied, multicoated Cosina/Epson/Konica accessory finders that were coming out in the early 2000s, we lost track of eye relief.  Eye relief is directly related to eyepiece size and overall finder size, and so small, cheap finders often don't show their framelines unless you shove your eye right up into them.  Sometimes you make a tradeoff, such as trading barrel distortion for better eye relief (such as on the Kobalux/Bower/Avenon accessory viewfinders).  Some of the best eye relief you can get is on the Voigtlander Kontur, which was essentially designed for eyeglass wearers.

7.  Distortion. A viewfinder might be your window on the world, but it has no effect on the finished picture.  So distortion in a finder, even if great, should not be a show-stopper.  In most cases, brightlines will be just as distorted as the view, giving you the ability to align them to similarly-distorted objects in the frame.