I humbly suggest that you simply take it to a shop and pay them. It is not as simple as buying the freon and putting it in. It also can be dangerous, high pressure gas, if you don't know what you are doing.

Highly likely that you have an R-12 system. You can't buy it unless you pass a test on the Internet, Federal Law. Why waste your time. You need to find out where your leak is.
A shop will put in dye and try to find the leak.

If it's your compressor, or evaporator then it's a task far about your tool and skill set.

Steve
'92 300E Sportline 242K (This car has had 2 major A/C episodes at around $1,200 each)