Cheesemaking supplies in Pittsburgh metro region?
Indeed, check out Pennsylvania Macaroni Co. They make their own mozzarella so it wouldn't surprise me if they have everything you need.
I see on their website that they carry Junket Rennet Tablets:
You may want to email their customer service department first. They are very helpful and provide prompt answers: firstname.lastname@example.org. Alternatively, you can call 1-800-223-5928.