Paypal (and subsequently Venmo) offers it. I've "instant transfered" money from eBay sales right to my account. They do charge a 1% fee (up to $25) versus zero fees for classic ACH transfer. If I need the money right away, I'll pay the 1% but I usually can wait the couple days.

On the bank side, one of the main providers offering RTP payments to businesses right now is JPMorgan Chase. If you have a commercial account with them, you can originate RTP payments.