Native Alternatives to Guppies

[Ambaryerno]

So, I'm now the proud poppa of a baby California Red-Sided Garter Snake. Right now she's going through guppies at the pace of two per day.

I'm working on setting up a breeder tank to get my own population going so I'm not at the mercy of availability (I was surprised when I discovered that a lot of the stores have stopped selling livebearers the past couple years. Fortunately I found one store that still carries them, though it's about 15-20 minutes from home, and adds a couple minutes on my drive home from work). I'm also going to work on getting her on feeder mice once she calms down enough she doesn't spook when I wave a pinky in front of her. However I'd still like to use feeder fish as backup and for variety. I'm also planning to build a paludarium for her once she's big enough, and I actually have room (hopefully the latter within a year). The idea being to replicate the natural environment as best I can.

That brings me to a conundrum...

The most commonly available fish I'm aware of that would be close to what would be found in the snake's range is the fathead minnow (which does exist as an introduced species in California). However I know that any fish that ends up in the paludarium's water feature is going to get et at some point or another, and fatheads have the same thiaminase problem as goldfish (although if mice are her main food source and the minnows are just an occasional snack that might not be as much of a problem). Others I need to watch for because they'll just end up being a very expensive meal, or may harm the snake directly (so I'll be staying away from any kind of catfish).

So basically what I'd be looking for is:

1) Something either native to the snake's range, or "close enough" it could act as a substitute (IE, I'm using Silver Falls Dichondra as ground cover in the terrarium I have right now because the native Dichondra isn't easy to come by, but it's a similar enough plant).
2) Something that will breed readily either in situ or in a secondary tank (perhaps using the sump I have planned as a breeder, so it's tied to the same water system as the water feature itself) so I can replenish the population if necessary. ESPECIALLY if the fish itself is going to cost a good couple bucks each.
3) Something that won't harm the snake when it inevitably gets nommed.

Does anyone have any suggestions? Since mice will eventually be her main food source (assuming I can get her converted. And no one figures out how to make readily available feeder frogs a thing) would it be ok to simply use the minnows since she'll have an alternative source for thiamine?