(My) common sources for car weights:  salvage from ubiquitous medical facilities that have had lead-lined rooms for x-rays;  printing type (the best source because they are already perfectly sized);  lead flashing-  easy (easy!) to use and commonly available without having to melt, etc.; and mounds of foreign currency that are either impractical or impossible to exchange, e.g. Franks, Baht, Rubles, more Franks, Pesos, Francs, and Crowns (various).  I have not yet had to use car weights because of the ease of using the foregoing, but....I am prepared to do so!