Charlotte Pipe has thin wall listed as Sched 30, not Sched 20. You'll probably need to contact them to find a retailer/wholesaler that may stock these if you are looking for odd size fittings.
I've bought these fittings in the 3" size to make modifications to the DWV system in one of my prior homes; the entire DWV system of 3" pipe and fittings was all Sched 30. The local ACE hardware carried 3" pipe and fittings in Sched 40 and Sched 30. But, there were just simple elbows, wyes, etc. I think I needed some fittings that the ACE didn't have and then the rest I got at Menards or Lowe's.
I don't recall anything in less than 3" size however.