I don't understand your problem. You need a 25.8 post, and can have a Kalloy within your budget, though it may be a bit heavier than you like. (Otherwise Kalloy posts are fine). Or you can have nicer vintage posts for more money, but don't want to spend that much.
Are you here looking for a contribution to your "I want a nicer post for less" fund?
Seriously, shimming won't do you any good because you'll still be looking at a nice vs cheap dilemma except for 2nd class shimmed fit.
Make a decision between the Kalloy, or shelling out for the post you want, or don't ride the bike while you wait for a miracle buy of a nicer post at a bargain basement price.
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