Most pet store employees don't know - or care - which feeder fish do and don't have thiaminase. Freezing doesn't affect it either. It's easiest just to avoid the whole problem and switch your snake from night crawlers to the smallest mouse pinkies you can find.
While mine were on night crawlers I did get some liquid calcium and put a drop of that on the worm just before it was fed to the garter, to make sure the garter got enough Ca for bone development.