Anyone know where to get one? 30.9 to 31.6 is common but Moulton (and Brompton) use the weird-ass 31.8 size. There are really good 30.9 posts available for not much (much less than 27.2 or 28.6 posts, for which 31.8 shims are readily available).
I suppose 31.6 plus a coke can shim would be 31.8 - cans are usually 0.1mm. At least, I used one to get a JIS cup in an ISO head tube (30.0/30.2)