It can be a real challenge to find the right version for a particular router, with the feature set that you want. I have this one: Tomato Firmware v1.28.7633 .3-Toastman-VLAN-IPT-ND ND VPN
running on an old WRT54Gv2. I just setup the VPN server last night and now I have to get my laptop on a different network to test the connection.
There are some (somewhat vague) download instructions on this page: »tomatousb.org/forum/t-379538/new ··· n-builds
You'll need to browse the readme files to track down how to determine just which build you need for your particular router. It is a PITA but once you get it running it is pretty awesome.