U.S. Boats has a nice "Locking" pin. I replaced all the pins on my 2100SC with these. I also went for pins that are about a quarter inch longer than need be just to ensure that they are fully through the inverted "U". I still had problems with them backing through the white plastic until I took a steel washer heated it with a blow torch and dropped it over the pin and onto the white plastic. With a pair of plyers I pushed this steel washer into the plastic. Needless to say I took the assembly off the bimini to perform this process.
Now with the steel washer embedded into the plastic I do not have the problem of pins wiggling out. I stopped looking for a replacement for the white plastic part, but that's the real solution.