In the USA, the National Academy of Sciences published a study years ago about nutritional requirements for mink. The National Academy of Sciences includes some of the best scientists in the country, so their word is the best available. There's a table in this publication that lists many fish species that have been tested for the presence or absence of thiaminase. I'll put a link to the table at the bottom of this post.
If you also look on page 15 of the document, you'll find a general discussion about thiaminase. Freezing does not denature/destroy the enzyme. Thoroughly cooking does, but most people don't cook the fish they feed to their snakes. In this study, they recommend only feeding fish containing thiaminase to the mink every other day (if at all) to reduce its concentration. Since mink and garter snakes have such different passage rates, it's best just to avoid feeding your snakes fish that contain thiaminase.
As an analogy, many people smoke for years without
noticing the harmful effects, but that doesn't mean it's not hurting you.
http://newton.nap.edu/books/030903325X/html/64.html