I too am in idaho and finding aquarium supplies unavailable almost everywhere. where do you go?
what area of Idaho are you in? I'm up in Moscow so I have a couple pretty good options.
First off... for the moment I work in a pet store. Once we are back in business after our move, Palouse Pets usually has a pretty good selection including some more specialty fish and they do special orders. Pets Are People Too really only carries the basics and no marine, but again will do special orders.
But my FAVORITE place to go is Bob's place in Lewiston. It's a bit of a drive from Moscow (~45 minutes), but totally worth it. We visit when we go out to Lapwai/the reservation to see friends. Lewiston/Clarkston also have Petco and a couple other places as well.
All of the above have aquarium supplies. So too does Walmart for very cheap, but DO NOT BUY THEIR FISH UNLESS YOU WANT YOURS TO GET SICK- if you do buy from them, be very wary and make sure to really look at their health in the main tanks/ don't put any of their water into your aquarium unless you want to be very sad.
Those are your best bets in the pan handle area. If you are in the southern half of the state... dunno. Boise and some of the larger cities will have the norm: petco, etc... and I assume there are a few specialty shops in there somewhere.
You could also try buying online. I know there are a couple betta breeders in Idaho who are pretty good. My house is also a VERY BIG fan of The Betta Shop in MN ( www.mnbettashop.com
) because they have gorgeous betta, update all the time and will mail them to you. If you want something other than betta, I have had pretty good luck finding what I want at www.liveaquaria.com
, including some supplies.