You must buy it...
How to obtain WordPerfect for DOS
WordPerfect for DOS is no longer marketed by Corel. Replacement and upgrade copies of WordPerfect 5.1+ for DOS may be bought legally from AlphaBest http://www.alphabest.com/Wordperfect/index.html (if the AlphaBest site is down, e-mail Carol Reese thewp51lady@alltel.net for ordering information; she will forward it to the person who handles the orders). A license number for WordPerfect is required, and it remains slightly unclear whether you need a 5.1+ license number or whether you can use the license number from a later version. The Alphabest site lists the name, e-mail address, and phone number of a person at Corel who can clarify this. (You may also want to get in touch with that person and ask him to authorize AlphaBest to sell copies of WPDOS 6.x.)
WordPerfect 6.x for DOS is harder to find, but copies of WPDOS 6.x and 5.x are frequently offered for auction on eBay. When bidding on eBay, read the product descriptions very carefully before bidding; some items are merely books about WordPerfect, not copies of the program itself. Note that packages marked "upgrade" may be installed on systems that do not have other versions of WordPerfect installed; "upgrade" packages have the same content as standard packages, but were priced differently.