Looking for a longer seatpost but the measurements i'm looking for are pretty obscure, wondering if anyone knew of a place I could buy this diameter. The seatpost fits into the tube via a small expandable bracket which allows the pieces to fit (the seat tube has about 1/2 inch larger diameter than the seatpost).

Measurements: All made will caliper
Seatpost: .86 inches or 21.8 mm.
Seattube: 24mm or .95 inches

Anyone know where I can purchase a seatpost this size? Bike shops around my city all think I can't handle the measurements and ask me to come in so they can measure it themselves.

Bike is a circa 1985 Peugeot road.

Forgot to mention. The bike was made in Switzerland: I know this because when I was replacing the BB the threading was inverted on either side. Thanks in Advance

Are you using a digital micrometer and measure exactly? If so, remeasure again, but this time measure the diameter of the seat post and shim as one unit. Then ditch the shim and just get a post that fits your seatube. You would be best off going to a bike shop, they have a tool to measure the inside of a seat tube exactly, and can tell you what size post you will need.

Bike was very likely not made in Switzerland. Peugeot made bikes in France, and for a couple of years, they came with Swiss threaded bb. Its a curse.... Motobecane did the same thing, only Motobecane did it for many years.

Its kind of like English threaded bb. Bike might be made in China, but it will come with english threading.

Thanks for the advice. I have determined it the seat tube width is exactly 24.8 mm. I know I will never be able to find this size so I will purchase a 25 mm seatpost and have a machine shop grind down .2 mm.
Thanks again

Well, you can always get some generic smaller size, those shims come in all kinds of widths. Just get a seatpost/shim combo that gets you to 24.8mm.