If they're sold as frozen fish food and say "silversides" they are arctic pond smelt. They aren't true smelt and sometimes the ones sold as food for people are called "silversides" but they are true smelt and contain thiaminase. The only place I know locally to get the Sally's is Petco and they aren't cheap, but still cheaper than live guppies. To make them go a bit farther, you could feed mostly tilapia but throw in a few whole silversides while you're at it.

You can try petco/petsmart or any place that sells aquarium supplies and live fish.