It's not J2 but 2J, Chapparal 2J to be precise. I think it appears in the used car dealership, but you'll just need to keep checking. I used a bought '70 Dodge Challenger with race modifications. It's only doable in the Dodge if the 2J and the Toyota 7 don't start at the front.
The other easy win car for the historic series is the Toyota 7 and you can win it from B-spec Like the Wind.
As for the DTM car, they (BMW 320GTR, Audi A4 and TT, Alfa 155, Mercedes CLK DTM, Astra and Calibra) all appear in the used car dealerships at some point. I used the Lexus ISF Concept that is buyable from the Lexus dealer. It is eligible (for some reason) and can win the championship stock.