First off, you should really consider waiting and/or saving money to prepair for a project like this. I personally did not build mine but know ALOT of people who have. For an average income person (35,000 - 50,000) it takes ALOT of money and time.
Hey, I really don't want to discourage you, I just think it's best to prepair and evaluate something before jumping into it.
It usually takes about 5 years (give or take a few) for a kit to be built start to finish. The money involved can run into the 25, 30, $40,000 range and even more depenting on detail. Most kits you se for sale on ebay or ki car webstes are approx 80% finished because the builder ran out of money, or his wife was complaining, or he has a new baby.
Plus, insurance on something like that would be virtually unavailable for someone under 25. I don't think it would be possible for a teen to get insurance on a car like that.
The best advise I could give would be to SAVE YOUR MONEY and get you something already built when you get a little older. I have seen some really nice used Ferrari kits go for $10,000.
To answerr your question about finding kits though, try a search on the net for "Kit Cars". There are about 6 sites that sell used kits.
www.kitkar.com www.kitcar.com etc...