I do PHRF on Tuesdays send me a PM when you are in town and I'll most likely be able to find room for you. If it's last minute just show up at Newport YC around 5PM on a Tuesday and ask around for the crew of Epiphany.
Wed. there is Shields OD best place to find a ride would be Ida Lewis YC
Expanding on JeffR's post (idk about the PHRF setup):
There are almost always open spots on shields for Weds. racing, so just go to Ida and talk to people on the dock. Generally people get there at 415-430 and then the launch brings people to their boats.
I'm sure the J/24 situation is similar (only raced there 2-3x so am not quite as familiar with it). Everyone has to use the cranes and then all the boats are tied to the docks while people rig up so just ask around there. Timing for J/24 is closer to 515.
Most crews will know if one of the others is looking for someone (in both fleets) so if the first person you find is set, they should be able to point you toward another owner.