You may want to look into Great Point Capital. A trading buddy of mine mentioned them as a good firm to trade with awhile back, although I don't have personal experience with them. However they are a FINRA firm and require a Series 55 and Series 62, instead of the standard Series 56.
From their FAQ's...
"Dark Books are becoming an increasingly important source of liquidity. We are continually looking for ways to access that liquidity. Currently, Lightspeed offers access to Credit Suisse’s Crossfinder and Goldmart’s SimgaX as gateways to dark pools. Takion accesses dark pools through established ECN routing strategies as well as Knight’s suite of algorithmic tools."