Where/how people find minerals
In the United States, mining minerals is similar to mining for precious metals (for the most part). You can establish a mining claim on a likely spot of Federal/State land and go to town. There are rules and regulations that guide the appropriate ways to establish, work and maintain a claim. You can also negotiate with a private land-owner for rights to mine on their land.

One step up is wholesale markets that sell mineral specimens (and all the items that go with them). There is a huge one in Tucson AZ in Jan/Feb with an amazing spectrum of stuff.

These markets are where most small retailers go to get their product.