A couple of suggestions:

1) This product was originally used by trucks:

http://www.c-tech.co.nz/producttrack.htm

2) Most companies that extrude masts also make just track extrusions. Here is an example
http://www.ullrich-aluminium.co.nz/pg31_32.htm

3) Aluminium extrusion companies sometimes have a standard product that will work. Try contacting local firms. I remember a story about someone using an extrusion made for screen doors

Both my examples were New Zealand Companies. I have had good luck dealing with New Zealand firms via internet

Good Luck
It sounds like you are into the hard part of your boat building, finding all the littole parts you thought would be easy