In environmental consulting, most of the gear is paid for by the company: the equipment, the supplies, the use of a vehicle for fieldwork (or reimbursement for using your own - my least favorite option). The use of specific safety gear is also included, such as steel-toe boots.
When it comes to more personal stuff, however, who pays for it? I realize that boots are about as personal as you can get (nobody wants to share your nasty, broken in boots, whereas jackets are more interchangeable), but they're covered by regulations. But what about other clothing?
Raingear: in the various places I've worked, I have not actually requested a rain jacket/pants etc. But I know coworkers who requested (and got) raingear. Was I just a chump for buying my own stuff?
There are 2 advantages to buying stuff on your own: first, you can get what you actually want, rather than, say, using the common drilling company method of buying everybody $6 rain suits that rip the day you open them (if not the hour). Second, if you buy the stuff on your own, you can use it for non-work related activities and not get hounded by management to return it on weekends and such (true story).
General field clothing: I don't know anyone who has tried to expense clothing. However, after an unfortunate project involving corrosive chemicals and the destruction of two outfits (significantly more than little acid spots), I was seriously tempted to charge my employer the replacement cost of my $75, hard-won field pants.