I've had really good luck so far with two web dealers. A google search for either will get you the websites if these are incorrect.
Midsouth Shooters Supply (
www.midsouthshooterssupply.com) really good prices on Lyman Great Plains Rifle kits,
and Track of the Wolf (
www.trackofthewolf.com) for supplies to finish the gun (barrel brown, stock finishes, balls, patches, lubes, etc.).
Shipping from both sites is reasonable, and the customer service from both is top notch. Whatever one doesn't have, the other will.
Good luck with your project. I'm in the beginning stages of a new .54 cal percussion Lyman GPR myself.