eBay would be the obvious answer, but the prices can be errattic. It's probably where you will find the best deal online, although I know it can be a hassle to go the ebay route sometimes. There are a couple sets up on ebay right now that seem to have a reasonable buy it now price. Not sure where you're located, but you might want to check any local swap meets for used stuff. A number of the local Harley shops around here carry them, but they've always been too rich for my blood.
http://cyclexchange.net used to have some 1" z bars, but the only type they have up there now are 7/8s. They might have some that are not on their website though, you could give em a call. I've heard nothing but good about them.