Look locally. Check out your local retailers for mass produced High School Musical merchandise. This could include room decor, bedding, clothing, novelty jewelry items and many other collectibles. To check before you browse, search the website site of your local retailer, such as Target, to find out what they carry.
Check eBay or other auction or specialty sites. If you are looking for a special or discontinued item, look on auction sites for people looking to get rid of the High School Musical merchandise they have acquired.
Buy direct from the studio. Disney is the creator of High School Musical and not only has online shopping options, but also carries High School Musical merchandise available at all of their Disney retail stores.
Consider shopping for merchandise that reflects your favorite characters in High School Musical. Many of the stars of High School Musical have their own websites and offer show merchandise on them. Zac Efron's site is just one example to check out.