25.4-27.2mm seat post shim on a seat tube with 26.8mm inner diameter
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.