These two are the best sites I know of, for DIY flavors -- I like bullcity because they show you very clearly which flavors are potentially dangerous.
BullcityVapor
EcigExpress
Wizard Labs is where I
buy nicotine; great prices. They also have flavors, good prices but the bottles are very small, 8ml instead of the usual 10ml. Their shipping is superb; 3 sealed bags around the nicotine, so no leaking ever.
Wizard Labs
Andria
EDIT: You can get PG/VG at any of the above, but Amazon is really better; their shipping on such heavy liquids doesn't gouge quite as deeply. I get the "Nature's Oil" brand of both PG and VG, and it's very good quality.