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I've found a few good sites, through years of digging.
Epilogue is a wonderful collection of artwork, generally very high quality. I know I've seen a lot of work there gracing the covers of various d20 books.
Elwood probably the site you've heard of, and, yeah, it does have a lot of so-so work. But it's also got a lot of really good stuff, s'long as you're willing to dig.
Illuminator's Guild has a number of nice pieces, too - of all sorts of styles. Most of the artists here are "for sale," as it were - I'm currently comissioning a set of pieces from one.
I usually just find an actor or actress image gallery site with a good clean headshot, without any grossly anachronistic (sunglasses, cell phones, etc) elements in it, resize it in a graphics program, and incorporate that.
Neverwinter Vault, they have alot of good stuff in there though mixed with some not-so-good. Like the bargain bin you kinda have to dig around a little bit but it's worth it.