Need Substitute for Double D Shaft and tubing

[blam]

I am building the english wheel described on this site. I am having a hard time finding a Double D shaft and tubing. Does anyone know what I can use to substitute for this?

[Bruce Dorsi]

If you are near a farm-tractor supply store, many stock Power-Take-Off shafting and tubing.

Some shafts and tubing are square, but most are rectangular.

[unklian]

I wrote about using a PTO coupling in a
compact lower adjuster for an English Wheel,
on allshops.org and chopperweb.net .

Part numbers and prices were also included.

[unklian]

http://allshops.org/cgi-bin/community/cforums.cgi?forumid=109061855423990&postid=9990208234249&action=messages&threadid=9990208234249&