Hi
Regarding S7 wheels, you can buy them directly from Slater's. All the main styles of wagon wheels are available to the S7 profile and back to back, as well as the coach wheelsets. Loco wheels are a different animal. The Group is working with Slater's to develop a range of suitable driving, carrying and tender wheels for a variety of steam outline locos, which will only be sold through the S7 Group Stores. The other method is to have normal FS O wheels reprofiled, either yourself (the Group can sell you a profile tool) if you have the relevant lathe skills, or by the Group (there's a service run by one of the members, at reasonable cost). If it helps, I've been a ScaleSeven member since the late 1980s, and never once had to run a lathe in anger!
The Group's web site has a fair amount of technical info scattered about, but I agree it can be hard to find answers to questions if you don't know where to look. The best bet is to contact any of the committee members and ask!
If you visit any large scale shows, look out for the S7 demonstration stand. The guys who man it are always friendly and willing to answer most questions people might have about the standard.