Contact large international auction houses such as Sotheby's or Christie's, or local auction houses, for artwork from well-known, established, or deceased artists. Use their knowledge and resources to inform your decision, but ask closely about their bidding process and any other requirements they have.
Reach out to independent art dealers near you. Explain what kind of art you want and they will use their expertise and connections to find what you need and to guarantee authenticity. Expect to pay high prices.
Contact art galleries, which work with artists directly, and purchase work from them. Use the gallery's resources; they build their business off relationships with artists and should be able to meet your needs.
Search off the beaten path for other sources of original art. Artists may sell work at local craft fairs in your area, or have small galleries of their own. Ask the artists directly where they sell their work, and frequent those shows and events.