i've had a cinelli XA 26.4 stem with 26.0 bullhorns for a while now, and haven't found a shim that holds the bars firmly enough to put all my weight on the ends, i mostly use the bars near the stem and the shoulders, so it wasn't really a problem. (i only have one 26.4 stem, but it's in a length i like) tried beer cans. someone said sparks cans were .0035" thick, so i went to the hardware store for something thicker. ideally .01" (.01"*25.4=.254 mm, .254*2=.58mm which is about the difference between 26.4 and 26.0).
they had a whole section of shims, strips of different metals in different widths and thickness. from .012" to .05. i ended up getting a strip of .018" stainless steel and a 6"x12" sheet of .033" brass perforated with 100 holes per square inch. i figured the holes would add some friction and keep things from moving. the perforation unfortunately tripled the price versus just a plain sheet.
i decided to use the brass sheet, and cut it to size. it was ***** to get in between the bars and the stem, especially since i didn't want to have to rewrap my bars, i kind of slid them back and forth trying to wedge the shim in there. i had to use a screwdriver to force the stem to open a little bit, since i was basically now trying to fit 27 mm bars into the stem. after tightening everything down, it seems good. put all my weight on the ends of the bars...solid.
between the brass sheet and the tin snips i got, it wasn't really worth it, i could have gotten a crappy stem that fit for that money. sometimes you just have to experiment.
eventually, all my stems will actually fit the bars they're on.
if anyone wants a shim from the perforated brass sheet, PM me. i'll drop it in an envelope and send it over.