In this case, I'd recommend your LBS. A wear item like a cassette won't be that much cheaper elsewhere, and you'll get valuable advice.

I'm having a hard time finding a reference for it, but I'm fairly certain that your RD will take up to a 28t, and likely a 30t. Your LBS should be able to find out for sure, and in the very least, slap something on and see if the B tension screw can be adjusted far enough out to work.

peace,
sam