You can find the Encyclopedia of Chess Variants in many online bookstores or large libraries. Some well-known online bookstores such as Amazon, JD.com, Dangdang, etc. provide purchase services for such books. In addition, you can also try to find it in local bookstores or second-hand bookstores. If you want to find a more professional version, you can look for the official website of the International Chess Federation or chess association, which usually provides relevant publications and reference materials.