No Cold Air
 

I have a A/C unit which is manufactured by GOODMAN Manufacturing. Last year I replaced the capacitor with a used one and it works fine. Yesterday I can't get cold air. I thought it is the capacitor again so I bought a new capacitor and replaced the old one and still the same is not getting cold air. I suspect its low on freon. If I can charge by myself where to buy freon or is there any special tool to charge a freon. Need help. Thanks in advance.