Get in touch with your school yearbook adviser or alumni association and ask if they still have copies of old yearbooks. Most schools keep leftover copies. Availability depends on the year it was published, while the price depends on the yearbook adviser's discretion. If they do not have extra copies, most schools keep yearbooks in libraries, so you can still visit and look at it even if you can't purchase one.
Look for the publisher of your yearbook in case your school suffered from a natural disaster and was not able to salvage anything. The publisher might still have copies stored somewhere, or a digital file it could use to print a copy. Ask your school for assistance and permission for printing.
Search for websites that offer services for locating old yearbooks. A number of websites offer such assistance. eBay is another source for old yearbooks your former schoolmates want to dispose of.
Put an ad in the local papers where your high school is located. Chances are, old schoolmates or school staff kept copies of your yearbook and can sell them to you. If not, you can at least look at them.