Air conditioning

Does anyone out there know of a mechanic in the Cedar Rapids area who can add freon to an older model car? We have a 1991 Olds Cutlais Calaiss with just under 52,000 miles on it. Bought it a year ago. We know someone who has some freon he has never used, so that's not a problem. The problem lies in getting the freon into the car. Don't know how to achieve this or if it is even achievable. If anyone has any anwers or suggestions, please let me know. Thanks.