is there anything alive and small enough on wild lake plants that betta frys could eat?

Answer 1

Yes there is BUT....

There will not be enough on even a huge amount of plants to feed a tank of baby bettas and further more, you run the risk of bringing in diseases and other unwanted animals. Far better to leave the plants from wild sources out of the fry tank.

MM

Answer 2

Probably not,however, there might be something large enough to eat Betta fry. It's late now,but try to learn how to culture infusoria,the bbs are too large right at first. Until then get a good quality linen handkerchief and squeeze hard boiled egg yolk through it,this will create fine enough particles to be consumed by the fry. You can but the kind of linen hanky that you need at a store that sells crocheting supplies.(The notions department.) If the material is not good quality the particles won't be small enough.
Happy trails.