I use the Tenenz folders myself; I get the top-staple ones. The first couple of years it was a bit of a topological exercise every year to re-remember how to staple in the pages so they come out correctly once all the folding is done - but now I have it down. I really like them.
If you're looking for customized folders with lots of options, I got a solicitation from Binding Systems of America. They'll make them with any spec you want; paper color, weight, style, texture, embossed, personalized, staple top, clip top -- you name it, they have it. And you pay for it. The pricing was more than Tenenz and the minimum order was _large_ (500, if memory serves). The Tenenz folders you can buy in packs of 50.
I also once used plain old double-pocket folders from the office supply places. Gave that up one year when I couldn't find solid color packs; just variety packs, and I thought that looked a little cutesy rather than professional. But they worked well.