It depends solely on the Clearing Firm....we use Goldman, and they make deals with Foundations, Pension funds, etc. to basically "rent" stocks by the year...and, of course, since they are an active trading firm, they have a lot of inventory as well. I have been surprised when they allow some obscure OTC on the hard to borrow list to get an "over-ride" and be filled (mainly on my pairs). Don
Shorting stocks has a lot to do with the desk that handles short orders ("LOCATES"), as well as how early you place your short order relative to everyone else. A stock like FFH, will only have so many shares to short, NO MATTER WHERE YOU TRADE. Actually, I find Penson pretty good. They are better than SLK in my opinion. Also, I trade with another firm that has electronic locates to multiple firms, i.e. GS MSCO LEH etc...