Still the same list of recommended non-bulge bracket PBs above. Besides that I forgot to name FC Stone, which also has very reasonable commissions.
A shameless plug: If you can afford to wait a bit, we (
Databento) are about to launch historical and live data APIs for all lit US equities venues and non-lit, TRF venues, around May. We source directly from the prop feeds instead of the SIPs, meaning we expose odd lots, auction imbalance, and all book levels i.e. non-protected quotes for all traded securities. I can only think of 3-4 other ISVs that make it easy to access the prop feeds, but they generally do so through software licensing and leave it to you to set up the multicast feeds, whereas we are licensed for external redistribution and handle normalization; other normalized options I've seen just distribute the SIPs.