IB was mentioned already, so you can check them up on your own.
tradestation claims to have shares of stocks down to $3.
cybertrader claims to have shares at any price; not on the super-hard-to-get stocks, of course.
if you generate enough commissions for your direct access brokerage, and show 'expertise' and that you're not putting the brokerage at risk, you should find someone there who's able to get you permission to short under $5 stocks. it was much tougher several years ago; you'd have to do it via a garban or prop shop only.