I've seen some pre-dyed ones at Lil Padded Seats
http://www.lilpaddedseats.com/store
and TieDyeDreams... pointed out in another post.
I would email the wahm selling the diapers and ask them... most would probably do it, but might charge a fee for an order under a dozen that they have to pre-wash. The reason they might charge a fee is that if they have to run their washer cycle a few times on hot, but for only a few diapers, it can get rather expensive... and they can't wash anything else with it, even fabric for pre-shrinking.
Good luck, and I know there are wahms out there who would be happy to do that for you!
nak - teri