Any good college bookstores nearby? That or an art supply house. Hard to judge them online. I need one as well and have been hemming and hawing getting someplace to check them out. Last thing you want are great prints in a crappy case.
I went out digging at lunch and found this one for a quick fix. $11.99 at Michael's and used their online coupon for 40% off of one regularly priced item, so $7.19 plus tax. It's a decent portfolio for a quick fix until I can hit the local art supply stores for something of a bit higher quality and/or order one online. MUCH better quality than I've seen some "photog's" present their work in, at least.
Includes 24 sleeves with acid-free black backing paper. It comes in all sorts of specific sizes, but this one was the closest on hand. A little work to center some 8x10's on the paper and I get a free black border.