Cherne test balls & plugs are what plumbers use (I was a licensed master plumber for 25+ years). They are not cheap, but can reach into pipes where it's difficult to fit an end plug, and will seal even if the pipe is somewhat rough.
Oatey 240018 Cherne 1-1/2-Inch Test-Ball Plug
Cherne 270024 Test-Ball 2-Inch Plug

Cherne 270016 Test-Ball 1-1/4-Inch to 1-1/2-Inch Plug
Here's a typical small plug:



But you'll need to look at the Cherne catalog using the product numbers, to be sure of what profile you're getting: http://www.cherneind.com/Pneumatic/Test_Ball/