where can I buy a cheap air compressor?

The Masterflow and the Slime are geared to inflating car tires and smaller items. They handle a tire in about 3 minutes. Great to have on hand for those items, but if you are in need of a more versatile compressor here are two very highly rated items. Both are on sale and both have free super saver shipping! Hope this helps:
DEWALT D55140 Heavy-Duty 1 Gallon 135 PSI Max Trim Compressor
Whether you're testing out plumbing lines or making quick work of construction projects with a pneumatic nailer, this DeWalt D55140 Heavy-Duty Trim Compressor offers dependable performance for improved job-site efficiency. Featuring a 1-gallon tank and 2.6 Amp motor that delivers .75 SCFM at 90 PSI up to a maximum of 135 PSI, high-surge performance, and quick recovery, this compressor is surprisingly quiet at just 69 dBA. Also offering fast, easy start-up, so you can get to work quickly, a high-flow regulator, and reduced breaker tripping, so you can complete work smoothly, this heavy-duty tool is equipped with a universal quick-connect point, so you can trust that it will be easy to set up with your favorite pneumatic tools. You can count on the durability of this unit with exclusive DeWalt details like an oil-free, long-life pump, which ensures smooth, maintenance-free operation that never results in oil spilled on sensitive work surfaces, and a ball drain valve that makes tank draining quick and easy. Cast-iron construction, a thoughtful roll-cage design, and a wrap-around control panel protect important compressor components from being damaged while you work. And at just 24 pounds and nine inches wide with a convenient cord wrap, this compact unit is easy to carry and store. This compressor is backed by a full 1-year warranty.

Porter-Cable C2002 Oil-Free UMC Pancake Compressor This highly portable oil free pancake-style air compressor packs a punch with an induction motor that provides longer life and better performance, whether you’re filling tires or firing a nailer. Complete with a built in automatic thermal overload protection, it can easily run off of normal household circuits and is able to run at 2.6 SCFM for faster recharge of its six gallon tank. The motor features replaceable brushes in order to extend motor life and 150 PSI max pressure to allow for longer run time. It also includes a High-Flo air regulator allowing more work at a higher pressure. Included with the C2002 compressor is an ASME safety valve, air regulator and gauges, 1/4 inch quick coupler, operating manual, and a one year limited warranty on compressor components.