I live in Boise...so here are some of my favorites:
-Craft Warehouse in Meridian on the corner of Eagle and Fairview streets (I work there) has some really fun stamps from lots of different companies. I really like the A Muse and Savvy stamps.
-The Stamp Dr. on Glenwood St. has some good stuff from Hero Arts and they make their own stamps too. Lots of shaped eyelets.
-Art Attack on Vista in downtown Boise usually has some fun things.
-It's All About Me also on Vista where it crosses with Kootenai. They are scrapbooking related but have some stamp stuff too.
There's a Michael's too if you want, it's on Franklin St.
HTH