Tower Hobbies, Walmart.com and New Egg have big RC semi-trucks for sale, as of 2015. Other places to find remote control semi-trucks include Horizon Hobby and TrendTimes.com.
Tower Hobby sells many different Tamiya semi-truck and trailer kits. Models include Mercedes-Benz, Cascadia and Ford Aeromax replicas with and without trailers. Tower Hobby is employee-owned and offers free shipping on most RC kits.
Walmart.com sells several Tamiya brand kits as well, including Ford Aeromax and King Hauler replicas. New Egg has one Tamiya RC truck model, the King Hauler, with the trailer sold separately. Horizon Hobby sells the Tamiya Freightliner Cascadia Evolution with the trailer sold separately.