Search for "whitebox" in the Server and Datacenter Products forum.
There are plenty of threads regarding this.
You can also look in the ESX 2 compatibility list. I have a HP DL360 G2 running with ESX 3.0.1 which is not on the ESX3 but the ESX2 compatibility list. Chances are good that it also runs under ESX3. Such hardware is available at affordable prices at ebay.
and even older HCL 3 systems (especially older G models from HP and FSX TX/RX) are available at affordable prices