I don't know if there is a decent US source for Lapua components and ammo.
In Canada all the Lapua bits can be had from Peter Dobson at email@example.com
I have bought from Peter a couple of times and he has been excellent to deal with. Prices are even reasonable for us Canadians and should be a steal for you Americans (I do not know if it is OK to import this sort of thing. Ask the ATF).