Stop! If your unit is running low in refrigerant there is another problem. The refrigerant that is used in your system is done so through a closed system. The refrigerant cycles back and forth between liquid and gas in a continuous loop. You should never run out of refrigerant. If your system is running low or is out of refrigerant than that means that you have a leak somewhere in your unit.
If you spend a bunch of money refilling your ac unit with new refrigerant but do not fix the leak than you will find yourself in the same position in the not too distant future. Fix the leak first, and then refill your refrigerant.
Be wary of contractors who need to ‘top off’ your refrigerant levels. Your refrigerant levels should stay constant unless there is a leak.