Ace Hardware if you have one local. McMaster Carr if you have to order. They are super fast processing and shipping.
Do be aware that allen head fasteners on a Weihrauch will require you to have 3 different sized wrenches on hand to remove the stock, tighten screws, etc. This drives me nuts personally, and I much prefer the factory slotted screws. Nothing worse than being in the woods hunting, or at a FT match and trying to find 3 separate tools just to check that the screws are still snug.
But most people don't seem to care, so carry on.