Is there anywhere else I can obtain a weight slip for my truck?
Yes. Aside from vehicle and metal scrapyards, you can also receive a weight slip from some auto parts stores and mechanic shops, especially if they specialize in modifications.
How do I go about getting my car registered in the commercial class?
After you have removed the seats behind you and replaced them with a permanent box or cargo rack, you should be able to register your car in the commercial class.