I don't think any or most US brokers allow individuals to short OTC stocks. Trding and others have posted some of their research (I believe Interactive Brokers was one that was mentioned), but I've never wasted my time trying to look myself.

However, these US brokers do loan shares to other investors who do short or provide access to shares allowing others to short, which is why some have posted here about their broker contacting them to borrow shares (FYI, most shares in margin accounts can be borrowed without explicit approval as it is likely in the TOS when signing up for a margin account). I've found monitoring iborrowdesk is a great way to keep a pulse on my long position in CYDY by trying to gauge short interest supply and demand.