Ron,
Just search for aluminum tubing. I got mine at Discount Metals, but they only have 2 locations.
A simple rectangle beam will still be $200 last time I checked.
Scott,
That must be the most expensive beam I have ever seen for kayaks. Hope you got it on special.
John,
Lots of designers don't use the simple beam, and there are good reasons.
However, I converted a Laughing Loon Northstar plan to Uncle Nicks plywood beam and it worked perfectly. It is probably reasonably easy to convert anything to the beam, if you desire.
Aluminum would have been even better due to its increased stiffness. I am just not ready to invest that much yet.

Although, I am thinking about building another Northstar. My only concern is that it is a little too directionally stable for me.

Marc