Where can I buy fine-threaded carriage bolts and lug nuts?

I dont see it? Most tire racks use a basic stud, bolt or other sort of fastener. If you want to keep the nut the same size, thats a good idea. But all you need to do is buy a standard course thread hex nut in the same size, ie, 3/4,9/16 etc as your lug nuts, then get the corosponding bolt size and go. Many bolts are cross drilled for a lock, so keep that in mind too.