I'm looking for artwork that has already been produced by the artist. Lots of artists have their own sites setup, but is there some kind of aggregator that has a bunch of them altogether?
You can do this on Photoshelter, although it is only images from photographers who use their service.
There is a shopping mode available in http://www.deviantart.com
You won't find an "aggregrator" of some kind because there are just too many services on offer in total, plus plenty of incompatible systems, add to that the fact that most services don't offer a godo way of interacting with another in the first place. (Short of post on Twitter/Facebook/Email links)
I.e. in the end you will need to look either at artists' sites or try a service that offers images from multiple artists.
500px.com has been mentioned above an is one site that offers print sales. fotocommunity.de offers sales for paid accounts, this is the German one but there is .com as well plus some more languages. whitewall.com is a print service with an art market that features select images. I think x1.com will get a shopping option or might already have one.
And I am sure there are many other sites as well.