Tim Wescott tim seemywebsite.com wrote:

....

If you have a scrap metal place near you, go check it out.
Clackamas Steel is a short drive from here, and in addition to
the big yard of rusty steel, they have a "brass shed" and an
"aluminum shed". There's a good selection of both machinable
and bendable aluminum in there -- my usual task on entering the
building is to restrain myself from trying to buy the whole damn
thing!

I got it all for less than $100, from McMaster-Carr.

aluminum, 6061, 8 mm Diameter, 6'
$4.04
aluminum, 6063, Tube, .065" Wall Thk, 7/8" OD, 6'
$9.17
aluminum, 6061, 1/16" Thick X 1" Width, 6'
$3.27
aluminum, 6061, 3/16" Thick X 1" Width X 6'
$11.00
aluminum, 6061, 1/16" Thick X 3/4" Width, 6'
$2.61
aluminum, 6061, 3/16" Thick X 3/4" Width X 6'
$8.58
aluminum, 6061, 1/16" Thick X 1/2" Width, 6'
$1.76
aluminum, 6061, 3/16" Thick X 1/2" Width X 6'
$5.97
Zinc-Plated Steel Threaded Rod 1/4"-20 Thread, 6'
$3.04
aluminum, 6063, U-Channel, 1-3/4" Base, 6'
$17.75
aluminum, 6061, 1/8" Thick X 2-1/2" Width X 6'
$15.97
aluminum, 6063, Square, 1" X 1", 1/16" Wall, 6'
$9.12
Merchandise $92.28
Plus shipping charges and tax.