You can get a recharge "kit" at most any parts store, even Walmart. But the little gauges are next to worthless, and you'll only know the low side pressure. To properly diagnose and recharge the system, you need a "real" set of gauges, a vacuum pump to evacuate the system, and the knowledge to use them. The propped way to recharge is by weight, not pressure.
Also, if you are low on refrigerant, you have a leak.