I got mine through Yahoo Auctions->Buyee. Expect to pay a couple hundred extra for shipping. This applies to software as well. When I buy software, I tend to spend an additional $80-$100 just for shipping.
The best version to get is an XVI. It supports switching between 10 MHz and the extremely fast 16 MHz. If possible, get a RAM upgrade module. You will need one if you want to load things from an SD Card (which you definitely do want). Similarly, try to get one that comes with a SCSI cable. The SCSI cable used in the X68000 is not standard so you'll have a rough time getting it working if it didn't come with one.
Try to get a monitor if possible. You can use it with 640x480 but ideally you would have a trisync monitor that supports 15 KHz, 24 KHz and 31 KHz. Without it, you'll need a list of commands to enter to load a lot of games and others might not be supported at all. I think an XPC-4 will solve this problem if you want to just use a modern display.
Make absolutely sure that you get a keyboard. It is a PC after all.
nfggames.com and gamesx.com are the sites I tend to visit when I need information. At this point, I'm probably an expert too.
Also, don't get into it if you are concerned about money. It's a sink.
Finally, don't buy anything I'm trying to get. I hate that.