I'd like to preface this by first stating that the best way to have it done right is to evacuate the system so that you'll know exactly how much R134 to put into your system and to remove any moisture that may have accumlated over time. Also he'll be able to pinpoint the location of the leak but as most of us know, it's probably the evaporator.

An alternative if you're in good terms with your mechanic is that he'll put the sealer in for you when he does the recharge. You can purchase the Interdynamic Recharge kit with a pressure gauge at your local Walmart. Also buy the Rubber leak sealer and Metal leak sealer. All this should be less than $50. The low pressure port is located near the passenger headlight in the engine compartment. I think it says 7 Nm or some other number. You need to remove this cap to do a pressure check and recharge your system. The Interdynamic pressure gauge may show that the pressure is within the normal cooling range but from my experience 1/2 lb of coolant loss will cause the system to cycle off and on and not cool as well. Be careful not to overfill, it's probably worse than underfilling.

The following is from www.ozbrick.tk :

Leak procedure
Most leaks on the 850 aircon system are in the evaporator core. This is very expensive to have replaced as the whole dash panel and HVAC has to be removed. It can be done by the DIY enthusiast, this is not a job for the faint-hearted! If the leak is a small one (most are) then it may be expedient to simply refill it periodically. In some countries, DIY refill kits are available. The system takes 1.65 lbs of R134a refrigerant.
The correct way to perform a refrigerant recharge is to extract and recover the refrigerant under a vacuum at a specialist shop. In some countries a shop aircon recharge costs only 5 dollars or so, it is not worth using a DIY kit. However, in countries where a recharge is expensive, cans of refrigerant and parts to top up the system are often available as a kit. Whether it is advisable to attempt a DIY recharge is best left to your own judgment after reading this article.

If the charge has completely bled away then moisture may have entered the system – this may cause trouble when recharging as the water can freeze inside the system and block it. However, if the system has only just lost charge, it may be worth trying to recharge it yourself as the desiccant in the accumulator bottle may have removed the moisture. If it has been empty for a long period then probably it is better to have the a/c professionally evacuated and recharged and a new accumulator bottle fitted.

WalMart sell an R-134a conversion kit which contains 3 lbs of R-134a refrigerant, l2oz of oil, can tap, hose, and also adapters to fit and convert old R-12 systems. No pressure gauge is included, so judging when the system has enough refrigerant is the key to success. If the system already has some remaining charge, this is not easy to judge. Other companies around the world doubtless sell R134a but their kit contents will vary. Often in countries where R134a is unavailable in cans it is because the cost of having it done professionally is very low – check around for prices.

Connect a can of refrigerant to the hose, then connect the hose to the recharge point, which is located in the engine compartment, under the diagnostic port. Place a thermometer in the outlet vent as described above. Start the engine (try to maintain at 1500 rpm), with AC on and set to coldest setting, RECIRC ON, Fan on 4. If the compressor does not start to run, the amount of refrigerant left in the system - and hence the system pressure - is probably too low to operate the Low Pressure switch. Locate the switch in the line at the back of the engine compartment next to the firewall. Remove the connector and jumper the two leads (green and gray). The compressor should now run. [Do not run it for long periods without a refrigerant charge or damage will occur to the compressor].

Keeping the can upright, open the valve on can to start the recharge. Placing the can in a bowl of hot water will speed up the process. The cycle time of the compressor on and off will get longer till it reaches its optimum point. Once the can is empty, stop the engine and close the valve to the can. Remove the hose from the AC connector on car and if necessary, replace the empty can with a new one. Add refrigerant until you believe the system is working correctly as described above, then stop. Too much refrigerant will cause the temperatures to drop too low, and the system will shut down under control of the high pressure switch for some minutes, then start up again. Be careful not to overfill the system as this can cause damage. If this is suspected use a test meter to see if the High Pressure switch has opened.

Normally, when a system has been leaking, the oil remains behind and it is not necessary to add more oil.
Some recharge kits provide a gauge to more accurately establish the state of charge of the system. To do this it is, strictly speaking, necessary to monitor both the high pressure part of the system (before the orifice tube) and the low pressure side, on the return from the evaporator to the accumulator. Unfortunately, the 850 has only the R134a refill connector, which is on the low pressure side, where the system pressure can be monitored. Normal operating pressure will be from about 40 psi down to 20 psi when the system is at it’s coldest, when the low pressure switch will operate and cut off the compressor.