The rear is the only really common failure:
Turner Motorsport - Item# 4111225649 - E36 Rear Subframe/Chassis Reinforcement Kit - E36
Looks like they have front kits now available too, though I don't know too many people who would have this type of problem:
Turner Motorsport - Item# TDR3675301 - E36 Front Subframe Reinforcement Kit - E36 Z3
And of course the rear swaybar mounting, which you shouldn't need if you pick a proper spring weight and don't put a solid oak tree in for a swaybar:
Turner Motorsport - Item# TSU3675001 - E36 Rear Sway Bar Reinforcement Kit - E36