Assuming you're going to mount the scope on the receiver, you'll want a rifle scope.
Any decent scope up to 9x would be fine for 100 yards. The best deals are often on 3-9x40 variable scopes, as they're one of the most popular sizes, but 4x can be fine for that range as well.
9mm has a good bit of drop at those ranges, so something with ranging features, like a mil-dot reticle, might come in handy.
Lots of people don't bother with magnified scopes out to 100 yards, and prefer red dots or holosights. I need some magnification for accuracy at that range, as my eyes aren't so great, and the 3-9x models are good for me.
If you're trying to keep costs down, Leupold, Burris, and Nikon all make good quality basic scopes that are below $200-300. Check
www.swfa.com for prices and models.
Quality starts dropping fast as you go below $200. If you have a bigger budget, there are lots of great optics out there.