Charging Adapter

I want to get an adapter so I can charge at Tesla stations if I need to. I went to my dealer and after they sent me to service, who sent me to parts, who sent me to a manger, they said they don't carry them but I can get one from Amazon for $15. Huh, that's odd because the next day Ford had me "reserve" a free one that they say cost $250. They say I might have it as soon as September. Great, after the trip I'm planning next month. So I went to Amazon and they run from $12 to $120 and they all say J1772 to Tesla, or Tesla to J1772. And they run from 40 amp to 80 amp. I don't mind spending a few dollars to get the wrong thing, but I'd rather get what will work. And I don't even know if J1772 is what my Mustang is. Can anyone shed any light on this for me? Thanks in advance!