The first step is to try and some research and see which connected with kit may be the right site for you. Do some research online or ask a person who builds model kits. If you do not know anyone the most convenient way is also included with online motors like google or look for online communities of model kit collector enthusiasts.
With that, the prices may vary widely. Some come with little to no instructions or help and some offer no guarantee's or money back guarantee. BEWARE there are even some kits out there that will even damage your engines. Some sellers of your kits are a regarding hype to obtain ?? to buy their apparatus. Using a fancy web page design as well as a sales page that draws you in with make purchasing. Some skeptics (there is always a few) even go as far to say the systems are broken at all, don't save or even gain fuel useage on vehicles they are installed on.