Most ghost shrimp available in the USA are
NOT brackish water species, they are freshwater. I've had ghost shrimp from a variety of stores over the last 5+ years and assuming they survive the first week or so (they tend to be kept in very poor condition by the pet stores), they usually live a year or longer. Almost always the shrimp being sold in pet stores as Ghost Shrimp are
this species.
I would call around to several local pet stores. Many sell ghost shrimp not as "pets" but as feeders for larger fish--two stores near where I used to live sold them 10 for $1 as feeders. You might have more luck if you call a few "mom & pop" pet stores (especially aquarium-only stores) rather than the big chains. You could easily add 5-10 without any worries about bioload.
Another option is to add a few (2-3)
amano shrimp. They get larger (a couple inches), will scavenge food and also are great algae eaters as well. You won't get them to reproduce (the larvae need saltwater to survive) but adults live in pure freshwater. Many pet stores will carry these as well, sometimes under the name Japonica shrimp instead of Amano shrimp.
Snails are certainly an option too, some can be quite colorful and add some personality to the tank. Apple snails, ramshorns (Columbian or European), nerites, MTS (Malaysian Trumpet Snails) are all possibilities. Some of those will eat live plants, so if you have a planted tank do your research in advance. MTS and ramshorns can multiply quickly especially if the tank is overfed. Nerites can't reproduce in freshwater so no worries about them overrunning your tank, but they are really more algae eaters than they are substrate cleaners. MTS, rams, and Apple snails all do a better job picking up uneaten food from the substrate.
Let us know what you finally decide/find!