Building and flying radio controlled model aircraft has got to be one of the most enjoyable hobbies around, and has been a favorite of mine for over 25 years. There are easily hundreds of companies engaged in on-line retailing of radio control model aircraft, aircraft kits, parts, accessories and radio control systems in the USA alone. Here are a few favorites from whom I've purchased electric flight related r/c items and planes.
Amain Hobbies, web site:
http://www.amainhobbies.com
Smooth web site navigation, good on-line ordering experiences, generally fast order processing and shipping.
Chief Aircraft, web site:
http://www.chiefaircraft.com/rcmsec/index2.html
Wide assortment of r/c aircraft, radio gear, hardware, support equipment, all at competitive pricing, Good customer communication via phone and e-mail. On the downside; shipping charges are often higher than those of other companies. Website order status info. lags behind, in my experience. I.e. an order for small parts and props arrived in 3 days (nice!) but the website order status info. still indicated "processing" the day after the order was delivered:-/
Extreme Flight Radio Control, web site:
http://www.extremeflightrc.com
Nice assortment of competitively priced high performance ARF aerobatic and 3D aircraft, both gas and electric models, available in a wide range of sizes from park fliers to large competition aircraft, plus hardware, brushless motors and electronic speed controllers. Fast order processing and really friendly folks to deal with. Excellent customer service.
Tower Hobbies, web site:
http://www.towerhobbies.com
Has been in business for over 40 years and made the transition to the on-line Internet world. Large product line, often runs sales, and has monthly specials advertised via their Tower Talk flier. Tower Talk is available by mail through on-line request.