I've run cultures from two different source ponds for a couple of years, one at 50' & one at 6000' elevation with major temperature differences. No idea if they're the same sp, or even native sp. - just fun
Where are you located? Most lakes and streams and even regularly flooded spots should hold, though presence of fish will really hammer population. If not amphipods you should be able to get daphnea and/or ostracods, which are smaller but also fun, nutritious and/or useful.
* They really like wood, so check under logs & put some into yoru culture tank. Grass & leaves are also good.
* They don't breed as quickly as you might expect, so as fish-food they're more of a treat than a staple.
* They are quick & good at hiding, so in the tank can become an ongoing "hunt" enrichment, particularly for smarter fish that get bored and/or stuff that's harder to feed.
* Also good in the filter (sump?) to keep things moving. Check
duanes
on this sort of ecosystems approach; maybe pester to do a sticky
PS: I'd suggest trying to find local rather than buying online, as there's a real chance that some might get though & into local ecosystem - best not risk introducing a non-native.