Sweet car! But please keep in mind that putting anything much heavier and/or bigger than stock on a BMW will wear out suspension & steering parts, wheelbearings, and brakes more quickly than normal (particularly the front lower control arms/balljoints/bushings.) They will also make the car slower than stock, and make it handle worse than stock.
Having said that...
http://www.tirerack.com has plenty of choices.
Good luck,
Jon