We've bought through three methods.
- Buying Supermicro built gear from RackmountsEtc (not much though).
- Buying bulk on corporate pricing directly through our Dell account manager.
- When we expanded in to the UK, we were able to order through the DC we are using as they had access to good pricing through their own account.
Different things work for different people. It really depends on what you want. I've known people buy old servers on eBay and colo them to sell them. About 7 or 8 years ago when I had my own business, I bought ~80 servers being cleared out by JaguarPC at the time and had them driven up to Chicago for colo. They were cheap to buy, the colo was well negotiated and we easily sold them. It was old stuff, but people still wanted it.