Bob,
I was faced with a similar dilemma with that problem. Not only that, but 'most' of the shims are not AL and therefore rust is a concern in your seat post. All I found was 'metal' shims, lol.
Kalloy has every size diameter and is nice thick AL machined tubes. I got a 400mm long one at a LBS, hack sawed it off to 300mm like my stock (surface rusty) post, so I could mark the safety line on it. And then I put my sanding disk on hand drill and rounded the edge corners off the sawed end nice and round while watching TV.
It turned out real clean looking once I loosened my seat post neck up and pryed it open with a screw driver. But once I got the 'round' Kalloy in there, it fit perfect. Of course that means a $5 hinged seat bracket is all I can use for now, but they are very strong (and brand new) and supports tilting and forward/aft movement. I would not consider one of the single bolt seat brackets with limited tilt, and would need 2 bolted seat bracket for my weight. Unfortunately I only saw the single bolted one piece post/brackets for my bike mainly (and there are dozens of them). The only one piece post/bracket I saw that I would consider with 2 screws was Thompson Elite for $80 minimum, but I needed a 'metal' shim even though they had about 20 sizes. So the handcrafted custom Kalloy was much better alternative for me.
But the Kalloy tube tapers down from about 26.4mm to about 20mm, so I can look for a dual allen screw seat bracket later. Kalloys machined AL looks real nice and shiney: $5 on line or $10 at LBS (shipping is only difference).
If you find a nice dual allen tiltable forward/aft seat posts, let me know :-) Those should all be pretty much standard diameters.
Thanks,
Phil Bailey