I agree with Scott on selling your home first as the builder will not pay market for your exsisting home. If the contract locks your exsisting home in with the purchase of the home you have contracted to be built, you could possibly lose both homes if you do not purchase the contracted home being built. Make sure you can trust the builder and check their references prior to hiring them.
Answering the original question, drive the neighborhoods you want to live in and call the builders off of their signs in front of the homes you see yourself living in.
Hope this helps,
Josh Barnett, Realtor
Metro First Realty