Determine the type of projects for which you will be using your airbrush. If you rarely use your airbrush, a small can of compressed air will suit best, and is disposable. These canisters do not provide much pressure (PSI), and are normally used for small, plastic model painting.
Look at an airbrush compressor chart. This will provide you with the best source of air for your airbrush based on what model of airbrush you have. These charts are usually available at arts and craft supply stores, or may come with your airbrush when you purchase it.
Determine where you will be using your airbrush. If you have a large garage or workshop, a large air compressor may suit you best. If you only have a small room with little ventilation, a small hobby compressor may be adequate. Hobby compressors are the most popular, and specifically designed for airbrush use. These are available at many arts and craft stores.