mcmaster.com
grainer.com
mscdirect.com
Use Lowes or HD to get your thread pitch, note the length and bolt head type.
If one of those places doesn’t have it, you don’t need it. No company selling bikes is manufacturing their own fasteners for stems, bottle cages, or common items. It’s from some huge industrial catalog. Sure, the Amazon/Ebay bicycle bolts might be wrong for your application. But they exist.