..

The instructions suggest hollow alluminum tubes, which should be light
and strong, and a good conductor to boot, but where can I find these?

Also, I've seen Yagi instructions that suggest a that the beams not be
insulated from the boom, and I've seen instructions that say they
should be insulated from the boom (or the boom should be an insulator,
like PVC pipe.) Does it make a big difference? Does it affect the
dimensions of the antenna?

You might try looking at a sporting goods store for the elements , that is
what the Arrow antennas started out as. Texas towers is a place you can
order from.

The instructions must be followed exectally at 2 meters and above. The
elements must be the same outside diameter. They can be solid or hollow
without a change in length. If the boom is a nonconductor , that must be
used, if the elements pass through and are insulated from the boom that must
be done. The boom will have an effect on the elements almost as the same
diameter as the boom diameter. While an inch or two might not make much
differance on the low bands, it could put a 440 mhz antenna out of the band.
You may be able to tune the swr to a low value but the patern will be way
off and the antenna may not have much gain.

The antennas are not that hard to build.