Ebay would be your main choice, thought I saw a couple on there now.
Many folks have put JBL 140's in the RH cabinets but some of the very knowledgeable people on this forum suggest they are not a good choice for a rear loaded horn cabinet. I have an old Utah in a BH cabinet and love the sound. EV's have been suggested and I must say recent experience has increased their standing in my ears. I believe Isaac suggested Eminence Deltas to someone else in this forum, do a search for 115RH and 215RH to find other discussions. Some of the discussions contain an amazing depth of knowledge about speakers, cabinets and how they interact.
A couple pics of my stuff: