I've seen some OV-15s show up from time to time before on eBay. But aside from that, Craigslist, Facebook Marketplace, and Habitat Restores are good places to look for lights.
Collector,Hoarder,Pack-rat! Clear mercury Rules!!
They're NOT cheap but here is an eBay seller that has some OV-50 silverliners. https://www.ebay.com/itm/Westinghouse-Street-Lamp/174152823303