Ask around for a guy that calls himself the Doctor. There may be as many as three of them out there, and they're all different iterations of the same person, from what I understand.

As for the subarches, I don't have an exact count to be honest. Probably around twenty? As for accessing them, unless they're locked you pretty much just have to find them, I think. You can make your own, too - but it comes at a cost. I don't know exactly what - I've never made one - but it can't be cheap, all things considered. It's a way to get a slice of home for yourself, and if there's a functional limit, I haven't seen it met. Someone I know brought his whole damned - and I mean literally damned - castle in.