I don't have one for sale but some tips from the pros that I read somewhere.
1.. avoid eBay if you can since you can not physically see the unit there might be issues with it or get damaged during shipping
2. Retrotech (YouTube) sell RGB monitors but only to his patrons.
3. Calling your local video/broadcast companies to see if they still have any..did not worked for me in my area.
4. Craigslist list Wanted post..post that you are looking for, long shot but hey.
5. Auction local surplus websites..I have see a few RGB monitors but things usually go high because everyone is bidding.
6. CRT Reddit - there is a listing of people selling and wanted every Friday.