If you are located in an area with a good electronics / computer culture, you can probably find used ones in local surplus stores. Here in Silicon Valley, that would be no problem.
As SirDice said, I don't think serial-connected terminals have been manufactured since the mid 1990s; they were all replaced by either Xterminals, or PCs running emulators. P.S. Just looked it up: As far as I can see, the last manufacturer of non-Ethernet non-PC terminals was probably "Boundless Technologies", which continued manufacturing Digital's VT3xx/4xx/5xx terminals after DEC quit that business (and quit being business in general).
I have two or three Falco 5220e terminals somewhere in my basement. This was a high-end VT200 clone. One sat on my desk for about 10 years; it had 3 serial ports (with speeds up to 19200 or 38400), connected initially to two VAXes and one cp/m machine, later all three to a RS/6000 (I had multiple windows, by using three serial ports). The fourth port was a parallel port with a local dot matrix printer, so I could get my own printouts without have to run over to the computer room. It not only had multiple display windows, it could also display both VT220 mode (in resolutions up to something like 132x43) and Tektronix 401x mode for graphics, so I could see graphs, and preview TeX documents. They have not been unpacked in at least 18 years, so I don't know whether they actually function. At least the two VAXes in the basement were brought up once about 10 years ago, and functioned (although the 4000/60 refused to boot, and the 3100 only ran when the graphics hardware was disconnected).
One can still get high-quality keyboards from that area, manufactured new. For example, there is a small company in the eastern US (spun out from IBM) called "Unicomp" that continues to manufacture IBM-style "clicky" keyboards, available in USB interface and both Mac and Windows layout. Those are the same hardware that was used on the later IBM 327x and 3290 terminals, and the original IBM PCs, with an incomparably accurate touch. I have two. For many years, I used them with a Macintosh, in my office (at IBM!).