I've linked this before, but
german kevlar battle bowlers are seriously worth getting from cheaperthandirt if you're in the states. They're advertised as Grade 2, but they're pretty much Grade 1. German helmet covers can be found pretty much anywhere, scour ebay/whatever for some cheap ones, the germans use(d) a lot of camoflauge patterns, so you're spoilt for choice. I picked up a second hand british Mk.6 w/ standard woodland cover at an insane 18 GBP, fellas from the forces sometimes sell their gear when they're made redundant, and more often than not it's at pretty awesome prices.