OTRCat is great. OTRNow is a bit better, I think.
Their collections tend to be a bit more complete, and they often combine shows with a small number of episodes on one disc. For example to get
The Creaking Door and
Dark Fantasy from OTRCat you have to get two discs but OTRNow combines them on one.
http://www.otrnow.com/store/collection.htmThere are several other sites I've bought stuff from; I'll look around for those later.
I love these old shows and often listen to them on my daily commute. Somewhere I've got links where you can get some of the harder to find stuff like
The CBS Radio Mystery Theatre from the 70's and the original
Hitchhiker's Guide to the Galaxy BBC series. If you're interested I look around and try to find them.
For the record: the
Quiet Please episode "The Thing on the Fourble Floor" is probably the most unnerving thing I've ever encountered. Here's an MP3 of it, but I'm not sure if this link will work...
http://www.quietplease.org/mp3/60.mp3EDIT: 'Tim's Old Time Radio' is another site where I've bought lots of stuff.
http://timsoldtimeradio.com/