Sportsmarketing used to make .177" plastic ammunition, in both ball and pellet, although they don't seem to still have them listed - only the 6mm size.
Bit difficult to say whether they'd be the answer to your problem anyway, though. Being so light they might not offer sufficient resistance in a springer to avoid the piston bottoming out, that is, not so different to simply dry firing it. And, if they don't crush or deform, firing them at a metal target (I know the one you mean - great fun!) probably isn't the best idea either.
Iain