Most plastic army men are around 54mm (1:32) scale which is too large for most wargame terrain (which tends towards 25mm - 32mm). However, you can play in 28mm scale or whatever scale you want while using the same rules! Below 25mm though you might want to adjust movement rules. If you want to stick to plastic soldiers there are a few options for terrain:
Kids Toys: Digging through thrift stores and rummage sales can turn up many play sets amendable to 54mm.
Wood Blocks: 2x4s and 2x2s which cut into 4" and 6" segments make for great terrain to climb on and hide behind.
BMC and others: Many plastic soldier manufacturers also make terrain that is in the right scale. BMC has this pillbox which is quite useful, but there are many more on the website.