relace your o2 sensor first...a cheap start

O2 sensors are known to last less than 10 years. If you exhaust looks really old/leaking, then maybe it is the catalyst covertor.
If I were you, I would buy a code reader ( $40), then replace the first o2 sensor ( the one you can get to easiy), chen erase the error code. Drive it for a week, if the error message does not come back, you are lucky. If it does, replace the next o2 sensor... and repeat.. If you go to a garage, they may do the same thing, or replace your catalyst coverter altogether. And if they don't fix the problem after doing all that--- you still have to pay them. You won't get your money back.
My 2 cents..