Here's a few other choices:
EliteFTS (EliteFTS - Powerlifting and Strength Training Products and Knowledge for Lifters, Athletes, Coaches, and Trainers
Rogue Fitness (Rogue Fitness :: Strength & Conditioning Equipment
Texas Strength Systems (Home
EliteFTS has top-notch gear, but you'll pay for it. Rogue is kinda middle of the road, while Texas Strength Systems is all about function without much regard for appearance. I've dealt with EliteFTS (for some smaller items, haven't bought a power rack yet), and rogue (bought shoes from them), but I have no experience with Texas Strength Systems.
Also, you may want to consider buying a power rack locally if you can find a GOOD gym equipment store -- on a power rack, you'll probably pay a significant shipping charge, which may well eat into any savings you get via buying over the internet.