I have a back-of-the-envelope quick way to do it that I use all the time on the fly. I go to Loopnet, find a property that is on the same street that is for sale or lease, and then typically the listing will have pretty good general info on demographics, traffic etc... Sometimes you have to pick and choose one or two locations because all listings don't have that info. But it is 100% free - and quick.
One good approach is the LoopNet data however cannot always find something close enough for comparison. Other approach we use is the Demographix iPhone app. I forget the cost but worth the fee as it can save you money in the long term.