Where can one purchase a onyx pendant?

Try a jewelry store like Zales, Kay, or other local chains. They will have the best selection and you can be sure that the quality is up to par. Macy's, Kohl's, or other department stores also have great selection and offer the convenience of choosing the best brand to fit your needs. Walmart and Target also have jewelry, but it's often on the lower-end of both price and quality, and there won't be anyone present to help you get it fitted or customized. Finally, you can try Amazon or eBay, but you can never be sure on those sites of the quality you're getting; still, they usually offer the best prices.