A handful of us in the East Bay of SF have access to a small family owned shop, Bowlin Bolts, in Berkeley that stocks an amazing collection of hard to find fasteners (SAE, metric and even British). Many of the bolts they cary are found elsewhere. However, they will sell at wholesale prices. This is often a small fraction of what you'd pay at the typical franchise hardware stores.
I recently found a site - https://www.belmetric.com
that specializes in Metric fasteners. They also have rubber isolators, hydraulic fittings, good quality hose clamps and lots of other automotive parts in metric sizes that I'm sure would be handy to anyone working on their G-restorations.http://www.siliconeintakes.com
is another that has been mentioned here before - hose clamps, vacuum tubing and boost hoses.
I'm sure others will have suppliers to add to the list.