here is the one everyone uses, particularly the folks that post on the messages boards that the OTC tickers can't be shorted https://www.interactivebrokers.com/en/?f=%2Fen%2Ftrading%2FViewShortableStocks.php%3Fcntry%3Dusa%26amp%3Btag%3DUnited%2520States%26amp%3Bib_entity%3Dllc%26amp%3Bln%3D