Well, if you're talking buy, not rent, you buy from the manufacturers or the retailers, there are many server brands out there, such as:
The Cobalt series - Sun Microsystem
Poweredge - Dell
There's also something called rack size, meaning how much space the server will take up, and it's based on height, U1 is one rack, etc...
It really depends what parts you put in there, such as the # and speed of processors, etc...