Is it the capacitor for the fan motor or the compressor? Some units have a single dual capacitor that takes care of both. Either way you will need to get one from an HVAC supply store. They are cheap. The last one I bought was around $7 or so. You will likely not find them anywhere else except maybe a REAL old fashioned family owned hardware store.
I do not know where you live but you may have problems even getting the equipment from the supply store. Some will only sell to licensed contractors. For little items like a capacitor they are not always as strict but it depends where you are. If you live up North or on either coast in union lands you will likely have to call a contractor, spend several hundred dollars for five minutes of work and a sub $10 part, and you will be waiting until Monday unless you can find one that does emergency work on the weekends....but expect to pay about double of normal price for the convenience.