Assuming that you have settled on which brand and model of air compressor to purchase, you can just type its name into your favorite search engine and see what kinds of stores come into view. Once you have an idea of which brands to look for, you should turn to your attention to finding an online air compressors supplier.
There are tons of places on the internet to shop for air compressors. The important thing to remember when deciding on where to purchase your new air compressor is whether or not you can get a better deal online or at a store. If you need your compressor right away, a brick and mortar store is your only option. Otherwise, if you would rather save a few (hundred) bucks, then you should seriously consider turning your attention to the internet suppliers, as you can usually save a lot of money through a ten minute search.