If you really want go cheap, find some construction sites where they are plumbing pipe that thick and go dumpster diving (or ask politely if they have any cutoffs). Those diameters aren't typically used in SFH, so you're looking for a commercial build site.
Typically anything shorter than a couple feet gets tossed, and you've said you need less than a foot.
FWIW you can get boatloads of rigid insulation scraps and < 2' lengths of framing lumber.