Hmmm...if you not looking for a particular artist, I usually bargain hunt at discount department stores like TJ Max. They have some nice pieces that are a pretty good deal at times.

EBay has a tremendous selection of art in all styles. Look for ones listed by the actual artist for the best deals. Naturally, many of these are done by unknown / undiscovered talent but if you find one you like it's like hitting the lottery. Sort of. http://shop.ebay.com

Yup, ebay is a good place. If you want something a little more edgy and unique though take a look at deviantART http://www.deviantart.com although I think they just do prints.

Depends on what reasonable means to you. Z Galleries has some nice large paintings http://www.zgallerie.com Crate Barrel also has some nice work, however the selection is very limited http://www.crateandbarrel.com