I buy uniforms on
www.rangerjoes.com - never had any problems with their service. I think you can find OD Green pants on there, but I would recommend a woodland camo for Riverside. Or if you want to spend extra Digi-camo is super cool and effective.
Hey - make sure you get Ripstop pants if you are playing paintball in them. I bought some cheap BDU pants at Dick's Sporting goods (I say cheap but they cost me $25) and they ripped up the croch from one knee to the other during Monster Game. It was freaking awful.
Ask anyone that saw my sewing and duct taping them up in the staging area.