Mmm...this is my first post here, so forgive me if I don't do this correctly! :-)
There are also sites out there (like
http://www.deviantart.com ), where some members post "stock" of themselves that ranges from extremely poor quality, to quite good--shots that may be full body, or face only--to be used by "photomanipulators". On dA, the stock is often specified as "only for use on dA"--but most of the people there are highly approachable, and would most likely be willing to have you use their images offsite, if you asked them nicely, and gave them credit.
http://lockstock.deviantart.com is a perfect example of a lovely person, with lovely stock images of herself--taken with home lighting--so you can have quite dramatic "before and after" shots. (While you're there looking--take some time to browse through the various categories of art--there's some breathtaking work available over there--but the photomanipulation work might be of special interest to retouchers! :-)) (Do bear in mind that people are more likely to respond to you if you're a member--and posting some work over there...dA, in particular, is VERY community-oriented!)
http://www.Pbase.com , is another place you might look. There are many people on that site posting portraits that are "straight out of the camera"--or darn close to it. Some of them are using low-end dSLRs, too--and many would be more than willing to let you "play" with some of their images, provided you asked nicely, and provided credit--or if you offered a trade of some sort--like some tips on how they could do what you do! ;-)
There are probably more sites like that out there--I'm just familiar with those two in particular.
Alternatively--take your own images, and work them up! :-) With Photoshop--any photo can look like it was taken in a studio (I even "paint" my own digital backdrops, in Painter!), or even the most washed out, overexposed image can become a glamour shot:
http://www.pbase.com/briarrose/image/38226161 http://www.pbase.com/briarrose/image/38211182 http://www.pbase.com/briarrose/image/42696400 http://www.pbase.com/briarrose/image/42696403
(And I agree that the other suggestions you've been given are excellent--but since they've already been made, these were the only ones left for me to use! ;-))
Best of luck to you!!! :-)