how to check & refill refrigerant?

I have the Trane XE1200 Air Conditioner, 4 tons, 12 SEER, model number
TTP048D100A0 since 1999 in Northern California for a 2 level 2100 sq ft
home.
According to its label, it shows refrigerant to be HCFC-22 is 8lbs 10oz?
http://www.angelfire.com/pro/young707/post.html
Can you please tell me where to get a meter to check the refrigerant level?
(any used one for occasional use, like once a year). And where to buy
refrigerant to fill it?
I was told that as long as my AC is giving cool air, its refrigerant level
is fine?