Where could I buy feather tails for topwaters? Are there any you can add your self?

Many of the larger tackle retailers offer these in various makes, models & colors.  Most are white. 

 

Here's one ~ http://www.tacklewarehouse.com/Mustad_Triple_Grip_Feathered_Treble_Hook_2pk/descpage-MDTB.html

 

A-Jay

 

This would work for poppers? Just replace the back hook?

They work on poppers and chuggers, minnow style baits and just about any bait that you pause frequently, giving the feathers a chance to pulsate.  They tend to inhibit the forward movement of walking topwater baits like Spooks, but when you want to keep them in a small zone, that is beneficial. I've seen guys use them on everything from cranks to 'stinger' hooks on spinnerbaits and spoons.