Here's a list of well known mail order companies that specialize in hobby and aircraft supplies (in no particular order):

List of manufacturers/builders that create and/or distribute sailplanes and other aircraft:

• Aerofoam Radio Control - Mark Mech's Combat Sailplanes
• BASH Enterprises - Home of the Mongo combat sailplanes
• DJ Aerotech - handlaunch and other sailplanes
• Dream-Flight - makers of Alula slope plane (and others)
• Kyosho - planes, sailplanes, helicopters, etc.
• ICARE Sailplanes - Sailplanes, Electrics, Handlaunch planes
• Phoenix Model Products - Manufacturers of Slope Sailplanes
• Pibros "Foamie" Flying wing - the Famous Pibros Delta Wing
• R/C Direct - R/C airplanes and sailplanes
• RnR Products - R/C aircraft, high-performance sailplanes, UAV/RPV, and other research aircraft
• Sky Bench Aerotech - Vintage sailplane designs from the 80's now being re-kitted
• Windrider Hong Kong - makers of unique electric and slope planes

List of manufacturers, distributors, or related components like radios, servos, electric motors, tools, etc.:

• Ace Wing Carriers - Airplane and Helicopter wing and flight bags
• AstroFlight Inc. - electric motors and controllers
• Banggood - overseas supplier of many related radio control parts and just about everything else
• Dremel Tools - Hand Rotary Cutting tools and more
• Don Edberg's Books on Programming Computer Radios - Programming the Futaba Series 8 Radios and more
• Drone Training Helpful Guides, Tutorials, Walk-Throughs and More - Lots of information about drones and photography
• Endless Lift (RadioCarbonArt) - soaring videos describing flying techniques and narratives
• Futaba - Futaba Radio Systems
• Hanley Innovations VisualFoil - airfoil plotting program
• Hitec/ Multiplex - radio control systems, chargers, planes, and accessories
• Horizon Hobby - JR radio control systems and other products
• Panavise tools - Specialized hobby tools
• Sanwa (Airtronics) - Radio Control Systems
• ScrollSander - small parts sanders
• Tom's R/C Electronic Regulator Circuit

Links to other pages that have R/C Sailplane links or other pages such as electronic projects:

• DroneNerds - Drones for professionals & hobbyists, repairs, filming, and production
• Drone Buying Guide - BestBuy-sponsored drone information and buying guide
• RCJudge.com - Information about R/C Sailplanes and other R/C hobby information
• Mountain Gliding Australia - Slope Soaring in Victoria, Australia
• Slope Soaring Australia - more about Slope Soaring in Australia
• A Guide to Building Model Aircraft
• Glue-It.com - Information about building all kinds of models, including shops, museums, and galleries
• GliderKing - Everything, absolutely everything to do with soaring (zillions of pictures)
• PicaSim - Flight Simulator for radio controlled aircraft - Great Sailplane Simulator with very simple controls
• Michael Shellim's R/C Soaring page
• The Torrey Pines Gulls
• South Bay Soaring Society
• Soaring FAQ - Frequently asked questions about soaring

Some Sailplane Videos:

• Glider Video Over Isle of Wight - A nice video filmed over St. Catherines Point