Garter Snake Care - gartersnake.info
This is the best site I have seen for care and feeding of garters. You'll want to read it because Goldfish are not recommended.
If your snake enjoys hunting a bit for his food, have you tried earthworms? You can get those pretty cheap at bait stores and if you powder them they aren't too bad for your snake. It seems as though pinkies are the best for him.
What about putting the pinkies underneath a log or vary where in the tank you put the food so he has to go looking for it?