Freon (actually it's not called Freon - it's R134A/Refridgerant) really isn't a consumable. Most likely your low on freon due to a leak or your compressor is malfunctioning. Unfortunately I am new to the Lincoln/Continental world so I am unsure of weak spots (if you had a Nissan I could list them off) for a leak.
Since your car uses R134A, it can be purchased at your local auto parts store (ie. Autozone). Most include a oil/conditioner that will help seal small leaks. But you will want to check the pressure in the lines first, so you will need to also buy a guage (although not always accurate, will give you a rough feel) used to measure the level of coolant. You do not want to over fill it, as running the air conditioning with too much coolant is just as bad as too little.
Hope this helps.