Here are a few online stores that have a good array of Powerlifting products and stuff that you won't find at most retailers. I have nothing to do with any of these sites except for ordering from a few of them, but since it is hard to locate some of this stuff I figured it would be good to compile a list:

EliteFTS - Awesome site run by Dave Tate. Carries hard to find items like custom board sets, sleds, and crap you never knew existed.
NewYorkBarbell - Nice overall equipment section and great service. Even have a strongmen section.
West Side Barbell - Alot of custom equipment made by Louie himself for WSB training and other powerlifting routines.
TheEquipmentGuys - From what I can tell they manufacture alot of the WSB equipment for other sites. Try contacting them for a direct quote.
APT ProWristStraps - Professional grade wrist straps and wraps.

This is short right now because there aren't alot of good powerlifting retailers. Please feel free to add to this list and I will update it the main post.