Fins Fins Fins
 

Does anyone know of a Fin only company, like Rainbow fins? Or a place you can have them made?

Future fins, FCS, but those are surf fins. Real surfing.

CDI fins... aluminum fins
http://www.cdifins.com/

I have seen CDI fins, they look great, but they won't work for cable.

Fins4sale.com
They have a bunch of cheap generic fins.

The general consensus seems to be that you shouldn't be riding with fins at all at the cable park. Regardless of whether they are plastic or metal. I always take mine off to ride there.

Yeah I am usually anti fin, I just have an idea for a fin, and want to play around with them again.

There are lots of fin companies out there, but most will not make fins/'new designs' for random people. Some will for a nominal fee. Rainbow use to make some good fiberglass wake fins, but they discontinued the line. Futures will make cut custom fins on their CNC (you will need to generate a CAD file) but they fit futures boxes (and it will be a $400 cutting fee). There are dozens more.

The easiest and most practical thing to do to start out is look at some cheap plastic or aluminum fins that are already drilled for your board template. Then you can grind and shape to what you want. Look at the site Mitch suggested, they are cheap black plastic and easy to reshape.