I third Atacom. We've been working with Jacky at Atacom for years. We also use Superbiiz (aka eWiz), Provantage, and Newegg.
Keep in mind that Supermicro doesn't sell ready-to-go servers. You either have to buy the parts to put it together yourself, buy one of their barebones models (and add the CPU/RAM/HD yourself), or buy a server from a company that builds them.
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We have got many systems from ASLab (also in Freemont just near HE) over the years. The systems on their web are really mostly supermicro systems built to order. They can also custom build systems to specs also.
They are a small outfit focusing on Linux but they have served us well over the years.
I've had good luck with rackmountsetc.com - configured and built a couple of supermicro servers for me a couple of years back, good warranty policy (replaced a couple of drives after 2 years, for nothing)