My aplolgies here, Rockyridge is correct. I assumed (and we know what happens when you do that) that you wanted a Hydraulic Thumb (much more useable than a fixed Thumb).
If you decide on a Hydraulic Thumb, I would add that there are more than one type of Hydraulic
Thumb-for the money, I highly recommend a "main pin" Thumb-it rotates on the same pin as the Bucket, and gives you much more rotation than a Thumb that is welded on the stick. The most expensive option would be a "Progressive Link" Thumb (gives the most rotation, but is overkill on a Compact Backhoe-and heavy). Remember, by putting a Thumb on your Backhoe you decrease the amount of material your Backhoe can pick up.
The other thing is that a well constructed Thumb will match your Bucket teeth (fit in between). The Manufacturer of the Thumb may ask for some dimensions to make it correctly (they are definitely all not the same). Just my .02-