I know FullTilt has it, but they are 100,000 min. HU SnG's, don't know if you have that ambition. Last time I checked they also had a King of the Hill type HU ring game but it was for the EXTREMELY high stakes play money limits - we're talking sitting down with 35 million, so you'd have to be pretty dedicated (probably not worth it). I'm not sure if they have added lower stake play money ring tables for HU, but you should check.
Never really saw the play money ones on Pokerstars, so can't say how much you need to play those.
is the easiest if you just feel like playing, but these are cash style heads up games w/ play money. You can play with the amount of chips they originally give you and can reload when you go broke.