I remember getting beta copies of OS/2 on their "Developer's Connection" CDs. Those days are long gone.
Up until a few months ago you could still buy Warp 4 from IBM. I don't see it listed now, but they still list the WSeB (server) product.
I think you'll have to go with eComstation or Ebay. IBM seems to have finally made it nearly impossible to get OS/2 from them.
Cheers,
Scott.