Petco and most other pet shops sell frozen silversideds, which is a whole fish that are about the size of a minnow and garters will usually take them with gusto, I would guess a ribbon would do the same, just put them in a small bucket of luke warm water to thaw them, then feed off tongs or leave in a small plastic cup overnight, should be gone in the morning.
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Jeff Benfer
gartersnakemorph.com