There are Several Excellent air rifle scopes, but keep in mind that many of them can cost much more than your rifle. Here are the names of just a few. Bushnell, Hawke, Leapers, Centerpoint and Gamo....
A 4 x 12 x 40 works great. Just make sure you can adjust the parallax down to 10 meters (or less) and that the scope is air rifle rated. The recoil of some air rifles will destroy non rated scopes.