Flint is often sold through 'sutlers' in the blackpowder-reenactor hobby. If you go to
www.ragweedforge.com and check out Ragnar's bookmarks, there will be several.
The so called 'primative movement' in which people duplicate older skills various native peoples abandoned ASAP for our more modern methods is another source.Try Wildderness Way, which I believe is online.
Finallly, you could visit Grime's Graves in England, a neolithic flint mining site, stip naked and hack away at solid chalk with dear antler picks to find the nodules.