I created this forum as a way people could let each other know about companies and mail-order distributors who specialize in OTR episodes. I recently found a great outlet which sells OTR epsiodes in MP3 format. This guy has a wide range of programs and most CDs have over sixty programs, plus he only charges $5 a CD. here is the URL: http://www.otrcat.com
There is of course Radio Spirits (http://www.radiospirits.com), who have a good selection of shows and offer free single cassettes with each order. Another one I found was Radio Showcase (http://www.radio-showcase.com). While they only offer single cassettes and CDs, they have a wide selection and (if you're willing to pay for it) you can have them custom made so if one of the cassettes offered has an episode you really want but the other one is one you have or don't like, you can have another episode from a different recording put there. They offer discounts for bulk orders. One disadvantage is that you can't order online and you have to send the order by snailmail.
I used to work for Radio Spirits doing audio restoration--the quality is good on their Smithsonian CDs and other box sets, as we cleaned the stuff up quite a bit, but their cassettes can tend to be dicey if they haven't gotten around to cleaning them up yet. Last I heard, though, they had 5 audio rooms working on the restoration stuff so they may have much of their catalog done.