The entire point of the socket 939 100 series opterons were to get away from single socket 940 boards. It is the current low cost solution for a single socket server. You can buy single socket 939 "server" boards on newegg. Just choose Server instead of AMD or Intel and you'll have a few options from companies like Tyan.
Here's your choices of socket 939 "server" boards:
link
If your hell bent on getting a 940 processor and buying registered ram, then get a dual socket board. This way you can alway pop another processor in down the line when SMP finally will start to affect normal users and you'll be set. You can't beat the price on 246s now anyway.