I've had excellent results from a small London-based printer for one-off or short-run vinyl stickers. Very reasonable prices, the quality is top notch, and the turnaround was very quick.
They did these mad max stickers for a project I did a few months ago.
http://www.oople.com/forums/showthread.php?t=38723
Anyway, the website is
www.diginate.com
I know that there are people on here that also do stickers, so I've been a bit hesitant to post this link. But hey, all I can say is that I've gone with these guys before and it worked out well.
Cheers,
Ty