I guess you should look in as many places as possible as decent ones are not common so you want to spread the net as wide as possible. If you are buying from fleabay or other non-specialist route then of course you need to take extra special care you are buying a decent one. And if I were doing that i'd probably invest a couple of hundred quid to have the car checked out by a decent specialist. If you come up on this or other forums with the reg plate then it might be the case someone knows the car and can advise on some of the cars history at least.
I ended up buying mine from Automart, but was lucky. I saw a few before hand which seemed OK on the face of it, but once I saw mine it stood out above the others as being obviously a far better car, and after 6yrs of ownership all appears to be well (touch wood). So if you buy a decent one it will be cheaper to run (minus mods) and more reliable than a Ford Focus, I know from experience, however if you buy a pup you'd better have deep pockets.