My voyageur 11.8 needs one, I think it originally came with an SR alloy one, but I'd take any cheap alloy one as long as it's quality. What should I buy and where?

Kalloy makes some decent ones that can be purchased inexpensively. Most any LBS should be able to order you a new one. Some might even have an old one in the back they'll sell you at a discount.

Kalloy has got you covered for 20$ Be careful not to strip the bolts and tighten them tight but don't over do it.

It won't have the smooth adjustment of the expensive Thompson posts but you can cut it to fit and reduce some weight.

I checked a few of the LBS' and none of them had any 26.6's. I saw the kalloy ones but I didn't know if they were good. A forum member actually just PM'd me and said he might have one, but if not I'll probably keep checking ebay.

If the other forum member can't help you out, I have a tall one that I currently have listed on Craiglist.

https://minneapolis.craigslist.org/bik/671597435.html

But you ought to be able to order one through any shop, or buy one from one of the e-tailers like Performance, Jenson, Nashbar, AEBike, etc.

Here you go:
https://www.biketoolsetc.com/index.cg...sts&tc=26-x-mm
I like these people.