Keep and eye out on ebay and RCU marketPlace usually I see a NIB one listed a few times a month and a few used one. Most LHS can still order parts for a re-build. Also check with Yellow aircraft and Top Gun Jets I had to order some supplies and inquired if they had any OS 91 in stock both said yes. They were a little pricey at 350-399.00. This was a couple of months ago, not sure if they still have anymore.
I recall that about a year ago, the OS91 was nowhere to be found...... and then they re-appeared. Perhaps OS has just temporarily stopped production again .... for a while. Bob