You can find them on ebay. Try searching "x inches tft lcd" (x being the inch value) and you can come up with some decent results. I found this one by doing a quick search:
It depends if you want a 4 or 16:9 screen, make sure you check the ratio.
EDIT: scroll down on the page I've linked to. You'll find all the size available. But if you want the cheapest available, you can search for a 7" rear view lcd. I'd still recommend the ones in the list because they're great quality and got vga input.