I buy my touch-up from automotivetouchup.com also.
I posted a whole series on using the Langka products for repairing those nansty rock chips on the front clip on the PTCW site. But, that site is dead for now.
The best part of the system is the Langka Blob Remover. That's the only thing I buy now - can be used with almost all touch-up paints.