For eBay, I just search "1u server" or a specific make/model ("dell r720") then filter down from there. Once you find a combo that works, then search by that specific model or view the seller's other listings. Many times they'll have pallets full of each type of server and may be willing to offer volume discounts.

The suggestion below to search for "1u server" is probably good to get a feel for what the overall used market is like and what relative availability is. It helps to keep up a little with the product line of whoever moves a lot of equipment in your area (Dell around here) so you'll know what the popular/good models are to watch out for when they hit the used market. It also helps to know what options are available and/or common for the model you're looking at so you know if you're seeing something that's specced up or bare-bones or in between.

Server manufacturers usually have someplace on their websites for hardware or technical guides or spec-sheets. They're often available for 10+ years after they quit selling a model, so you can research a system before you buy a truckload of them.