The only readily available 250w hwi ballasts I know of are the aquamedic reeflex. They say they are M80 although Advance (Philips) stopped producing these ballasts so perhaps aquamedic happened upon a stockpile.
http://www.aqua-medic.com/reeflex_ma..._ballast.shtml
Otherwise you try your best to find them used like I did. It's not easy!