There are some that can be used. If your garter snake is still small, you may want to try to get some pinkie slices into his diet. There are a handful of pet stores hat sell feeder mollies/feeder platies. These are hard to come by though. Below is a quote from the Garter Snake Care Sheet, which tells you which fish are ok to feed and are thiaminase free.



Ayu (Plecoglossus altivelis) Bass (largemouth) (Huro salmoides) Bass (rock) (Ambloplites r. rupestris) Bass (smallmouth) (Micropterus d. dolomieu) Black backs (pseudopleuronectes americanus) Bluegill (Lepomis m. macrochirus) Chub (bloater) (Coregonus hoyi) Cod (Gadus morrhua) Crappie (Pomoxis nigro-maculatus) Croaker (Micropogon undulatus) Cunner (Tautogolabrus adspersus) Cusk (Brosme brosme) Cutlassfish (silver eel) (Trichiurus lepturus) Dogfish (squalus acanthias) Eel (anguilla rostrata) Gar (northern longnose) (Lepisosteus osseus oxyurus) Haddack (Melanogrammus aeglefinus) Halibut (Hippoglossus hippoglossus) Hake (Urophycis spp.) Herring (Leucichthys artedi areturus) King whiting (ground whiting) (Menticirrhus americanus) Lemon sole (Psuedopleuronectes americanus dignabilis) Lizard Fish (Synodus foetens) Mackerel (Scomber scombrus) Mullet (Mugil spp.) Perch (yellow) (Perca flavescens) Pike (northern) (Esox lucius) Pike (wall-eyed) (stizostedion vitreum) Plaice (Canadian) (Hippoglossoides platessoides) Pollock (Pollachitus virens) Porgy (scup) (Stenotomus aculeatus) Porgy (scup) (Stenotomus chrysops) Pumpkinseed (Lepomis gibbosus) Redfish (Sebastes marinus) Salmon (Salmo salar) Salmon (Coho) (Oncorhynchus kisutch) Seabass (centropristis striatas) Sea catfish (galeichthys felis) Sea robin (Prionotus spp.) Smelt or "silversides" (Hypomesus olidus)(arctic pond. Also known as silversides, usually frozen available in pet stores) Spot (Leiostomus xanthurus) Squid (Loligo brevis) Tautog (blackfish) (Tautoga onitis) Trout (brown) (Salmo trutta fario) Trout (lake) (Christiconer n. namaycush) Trout (rainbow) (Salmo gairdnerii irideus) White trout (Cynoscion nothus) White trout (Cynoscion avenarius) Whiting (Merluccius bilinearis) Yellow tails (Limanda ferruginea)

Hope this helps!