I find them on EBAY from time to time.
The OV-10D is a favorite of mine and I get what ever I can on them from time to time. I just bought two 1/144 resin OV-10A's. Now all I have to do is get a milling machine and I can bore out the cargo bay on oneso my parachutist will have a place to sit. Best of luck on your search.
Tally ho
Rosie