Officially the M58 is the preferred ballast (see
www.ushio.com) for the Ushio 250W/10K but I've heard that people find better results on the pulse-starts ... which is .. um ... M158? Not sure offhand but yes yet another ANSI "M" designation. Although the M80 might still be the best bet since as Steve mentioned it's the closest to Euro specs.
If you REALLY want your head to hurt, surf on over to
www.advancetransformer.com and read up the specs on all their HID ballasts. If you start to get the run-around from lighting suppliers (I don't know why they do this but they all do it) it is sometimes helpful to tell them "look the part # of the ballast I need is 'Advance blahblahblahblah' so please look that up" and then they can't tell you anymore that the ballast doesn't exist, they have to come up with a more creative excuse not to sell it to you.