Dealer, some auto parts stores, Cummins, Auto Electric shops, and even heavy equipment service shops.
Just need to know if it's a Weather Pak, Deutsch, Packard, etc, brand and plug shape. Take a picture and bring it with you.
Also going to need to know what wire goes where.
Post your truck details (year, model, trans, etc. . ) and we might be able to tell you which is which, unless it's a simple 2 wire, then you are good to go either way, or you have a 50/50 chance of getting it right the first time.