25.4-27.2mm seat post shim on a seat tube with 26.8mm inner diameter From where I live (Philippines) , there seems to be no stock for 26.8mm seat posts. All other sizes are available except for 26.8mm; they don't even sell shims for that size. Currently I am forced to use a 25.4mm seat post on my bike.
Is it possible to use a 25.4 - 27.2 mm shim for that purpose? I've used beer cans and inner tubes for shims, but both options has led to the seat tube crimping as I tighten it down.
There are many different standards for seatpost size, and 26.8mm is one of the more-common ones.  Its not rare.
I suggest you buy the right-sized seatpost.  If your LBS won't order it, then look to buying one online.
A shim is a solution but as you found there are drawbacks too.  Bodging a shim from a can is really only workable for 0.2 of a mm at most.  More than that and you'd want a proper shim which is at least 100mm tall.