If your dash air is still putting out cool air, meaning that the system is still intact, just low on freon, I would buy one of the recharge kits at an auto supply house and charge it yourself. The kits come with instructions and usually include a gauge that gives you a green area that indicates when the charge is sufficient.
If you take it to a shop, they will probably say that you have a leak, which you obviously do have, and they will start listing all of the laws/rules, that they claim will not allow them to recharge your system without first repairing the leak. The facts are that 99.9% of all AC systems will eventually lose some small amount of freon through the years. The simple addition of a can of freon may be sufficient for several years of good AC service.
If your system does not have any freon at all,and only puts out warm air, then by all means take it to a shop, have the leak repaired, the system evacuated, and properly charged.