I think the best place is going to be E-Bay and buying one used. You can get the t5720 generally for around $75-$125. The two things to check when buying these units is the amount of installed RAM and the size of the flash drive. Both of these items are standard sizes and easily replaced or upgraded, so if you get a good deal on a lower spec'd model, you can get more RAM and storage.
Here is a good how-to thread on how to upgrade the units. Keep in mind that the thread was written specifically for blackjack users, but the information is accurate and easy to understand. I have upgraded the RAM and flash in some the units I have bought. Other units I bought had all that I needed and I've left them alone.
The hardware itself seems to be pretty robust. There aren't any moving parts or fans, so the odds of one not working are pretty low. Although I have seen a couple on E-Bay that clearly mention they don't work. Obviously a surge or electrical problem could take one out. I think most of these units on E-Bay are sold by recycling companies that pick up large quantities of these units. I generally trust those companies to give me an accurate disciption of the product because they don't want a bad reputation. So I say buy with confidence if you know what you are getting.
Some of the current units on E-Bay do not have any RAM installed in them. The discription says there is no RAM, and the picture also supports the fact that there is no RAM chip installed. Just make sure if you buy one of these units, you realize you'll have to buy some RAM to go in it.
Here is the How To Install CQC thread. It goes over a lot of these details and also explains how to reflash these units. It is a simple process and the image is available from HP. So if your unit doesn't come with an OS installed, or if it has been customized (our group buy were units from USAirways that still had their proprietary software on it), you can easily install the OEM XPe image without incuring any cost.