Marine Institute in SD is a good one. Back in the day, Chapmans in Stuart, FL was the go to. I spent 3 months there to get the my first license at the turn of the century. Great program but I think it is no more. I have taken several renewal courses at Marine Institute and they have been pretty good. Usually the biggest hurdle is getting the required 360 verified days at sea for the size class you are after. Get yourself (if you haven't already) a copy of Gonders (The CG License 6 Pac to Master 100 Tons), Chapman Piloting, and Dutton's Navigation and Piloting (which may be dated, but still good info). Getting the license is good even if you don't work it, just for the knowledge when sh*t happens....as many of you know already.
The above posts are spot on with Maritime Institute. Another good resource believe it or not is YouTube. There is a guy that goes over every question and answer. That helped me as well.
It’s a good course to get whether you wanna charter or not. Just because you have a captains license though doesn’t mean you are good to start charging people just FYI. A lot more money And regs involved for that.
If your going to do it, Don't waste your time on a six lic, It's a joke. Wait and just get the masters license. it is well worth it. You'll be sorry if you don't. I got mine 30 years ago and it's 100X better than a six pack.