My immediate thought was Old Potrero. And as already mentioned the 100% corn whiskies. Most whisky does contain malted barley for the enzymes, often 10-20% for that reason alone, so most rye whisky etc. will contain barley malt (rye mashbills of 95% rye are common, and the 5% will usually be barley). Look out for those that state 100% rye or 100% spelt etc.. A huge variety of single grain whiskies are made, however most will be small operations.
I'm not recommending them because i never tried it but here's an example http://www.koval-distillery.com/drink-b ... de-whiskey
There are people out there with barley allergies that can consume malt/barley whisky, maybe there is a safe tolerance level for yourself, you just need to find it. Either way research on most particular whiskies will reveal the mashbill.