Is your snake wild caught? I mean if it's from the area you are talking about and it's already been out there eating that stuff, then go for it. I believe that toad larvae do not secrete toxins. Wood frog tadpoles would be excellent garter food. Heck, they are easy and cheap to raise them up to little froglets too. I used to do that all the time with bullfrog and chorus frog larvae.

If your snake was captive born I'd say stay away from wild food but I've never had any health issues with wild snakes and I fed a pair of concinnus on wild frogs, larvae, and fish for nearly 20 years and never had a problem. If they had parasites they weren't doing any harm.