Dual Speed and Turbo Boost is the same thing when talking about Bit Char-G's. All it does, if I remember correctly, is provide less power to the engine until you hit the boost or action button and then it provides full power giving the illusion of boost. And you can get them at:
http://www.tinyrc.com
http://www.toyeast.com
http://www.cosmogames.com
http://www.hlj.com
Look for the Fairlady 350z or Auto Modellista sets, those are boosted. Alternatively you could buy a booster set (which is just an add on set with no controller...just comes with chasis, clear body, engine, gears, and tires) and the 4 channel multi-controller which is what I did. Then I just went and got the NSX-R Mobil1 body to replace the clear one.

Anywho hope this helps.
PS - Sorry...I can't help but be long winded...it's in my genetic make-up....