You probably won't find silversides at the grocery store... pet store or possibly bait store... but you can buy trout or salmon at the grocery store and cut them into thin strips for your garter. Buy a trout or salmon fillet, cut into many many worm sized strips, freeze on a cookie sheet and then put the leftover frozen strips in a ziploc bag and defrost a few as needed. The trout or salmon fillet may seem expensive, but will last one garter snake a very long time. As other's have said you should add some powdered calcium. You can find it at a pet store and you don't have to give it every time or even have it at all to start with, but it is important to get some eventually. Good luck with your new little one