Well, you'll need 26.4 handlebars if you don't wish to use a shim. However, if you are comfortable using a very thin shim, then you can use 26.0 handlebars (most standard road drops and bullhorns). I've run many shimmed setups without difficulty or slippage. A piece of cut aluminum can works quite well. I honestly wouldn't worry much about slippage issues. If extremely concerned, you could buy some .2mm shim stock (available at mcmaster.com and other places) and wrap it around the bar once to give you the .4mm that you need.