Hm, well, it tends to be the newer artists who take requests, so if you browse the art and find someone who is newer and less popular, you might be able to find someone. You can also certainly trying to search the submissions, ordered by date, for request or MLP request, etc., and see if you can find any folks who are submitting finished requests to see if they're still offering them.

Good luck!

Sorry, for some reason the FA parser broke my links. They should be:

http://www.furaffinity.net/search?q.....;order-by=date
http://www.furaffinity.net/search?q.....;order-by=date