After market pistons should be available - a good make like AE (or similar) are often as good as OEM (I don't think BMW make pistons anyway).
Good - Fast - Cheap: Pick any two.
3 x E32 735iL (Alpine White, Glacier Blue & Kashmir Beige); 2 X E38 735iL (Oxford Green & Alpine White); E53 X5 4.4i (Pearl Beige); R50 Cooper; R55 Cooper Clubman (Both Pure Silver).
E38 Parts Car
E21 318i; E23 735i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i
I've just changed my wiper blades and now my reverse lights don't work. Should I change my fuel pump relay?