There is a set on thebeachcats.com, but they don't have the beam inserts.
The word from Performance is (was?) that they have a lot of the wings, but the manufacturer of the rubber inserts jacked up the price too much to the point that they were unaffordable. Performance was working on a connction straight to the beams, but I never followed up on the status of that.
I had a set of wings for my Prindle 18.2 and they were pretty cool. The plugs were a little difficult to install, but once they were in, it was extremely convenient and I was very happy with them. I would recommmend riveting the wings together when installing, though.
If you think the price of the Performance wings is a lot, try comparing it to the Hobie 18 wings!!