They're literally just cans of air. Just find whatever you can get the most of for the least amount of money. Or buy one of those handheld vacuum/ blowers, more expensive than canned air but you'll never need to buy it again and it's better for the environment.
Yup, they're little handheld units. Some are vacuums, some are blowers, some let you toggle between the two. Just search up "PC cleaning vacuum" and you'll see what I'm saying.
I've heard that blowers are safer to use on PCs than vacuums, as using a vacuum can create static electricity, which computers don't like. I can't verify the validity of that, though.