Try LEF.ORG. Grocery store supplements are hit and miss in terms of strength and purity. LEF has been selling high quality supplements for years. In fact, they have certification labs that other manufacturers use to certify purity etc.
Life Extension is the best. I get almost all my supplements there. Expensive but worth it.
I also use kratom daily to help me stay focused. LEF doesn’t sell it. The FDA is trying to ban it.