Some 'pound shops' sell them. (I'm not sure about whether any of the 'big names', such as Poundland or 99p Stores, would be of any use but I know of several local independent 'pound' traders, or similar discount stores, where I've seen aerosol paint for sale).
I've found the cheap sprays to be a false economy, the coverage is sketchy and you need to buy more of them, though it depends entirely on what you want the paint for.