A hobby shop is a good source. Well-stocked hobby shops carry domestic and foreign kits, paints, accessories, tools, books, and magazines, such as Scale Auto Enthusiast. If you aren't near a hobby shop, you can order almost anything from mail-order businesses that advertise in Scale Auto Enthusiast and other hobby publications. Many overseas firms accept major credit cards such as VISA and Mastercard. If you don't have a credit card, or if a vendor won't accept it, you can order kits with international money orders available at post offices and banks.
Many large cities have modeling clubs.
You don't have to build strictly plastic models to become a member, though. There are hundreds of modeling clubs around the United States and the world. If you're having a tough time finding a club in your area, check with your local hobby shop. Several contests, conventions, and swap meets are held around the country each weekend. They usually include modeling seminars, museum tours, and other programs, in addition to vendors who sell kits and accessories.