Visit the magazine's website and visit the customer service page. Here you will find a link for purchasing reprints if available. An online store on the website may also contain back issues or reprints. This will vary from publication to publication.
Visit online auctions or superstores for available reprints. Publishers may put them online or others who purchased reprints may have these available for sale. When purchased through outside stores or auctions you may pay more money than the original sold for.
Contact the publisher directly. This information is available on the magazine's website under "Contact Us." If reprints are available or the publisher can make a reprint, you can discuss it through email or by calling the publisher.