Every town has swimming pool/spa stores, but they are usually small shops located in out-of-the-way places. So if you don't own a pool or hot tub, they are the kind of place you'd never notice! Check your local yellow pages.
Now that I think about it, Minnesota may not have a lot of swimming pools (besides the indoor commercial-scale ones). But I'm sure you have spa/hot tub stores, and you can call and ask if they carry pool filter sand.
(Message edited by jennewt on June 23, 2006)