I purchased an app in 2013 and I can't find it in my cloud. How do I get it without buying it again?

If it's the same app, same account, and same country (i.e. you haven't moved countries), it should show in the Purchased section for redownloading (Redownload apps, music, movies, TV shows, and books from the iTunes Store, iBooks Store, and App Store - Apple Support), if it doesn't check to see if you've hidden it from there : Hide and unhide iTunes Store, App Store, or iBooks Store purchases - Apple Support

Note that if the developer has pulled the app from distribution, it won't show in your purchased list any longer. You cannot re-download an app that the developer has removed from distribution.