At any dollar store, Wal-Mart...most grocery stores too I believe. They carry a product for ethnic women called Pink. The oil sheen spray that comes in the aerosol can - and the aerosol kind is the only kind that really does well - works absolute wonders on tails, and costs $2-$3 a can, lasts forever.
I would not recommend it right before you braid, because while it doesn't leave an oily residue, it does make it a little slick. However, regular use and carefully brushing out the tail can produce a soft and shiny tail that has wonerful texture beyond immediate application.
I have used it 1-2 times a week on Amber for 8 months, and her tail almost has the texture of human hair now, and is very strong and resistant to breakage.
Everyone asks what I use on her tail. I was lunging her the other evening, and when I brought her in, a visitor at the barn immediately asked what I was doing to make her tail so nice.
There is nothing better. I would never pay $15-$20 a bottle for the high-dollar brands. As with show-sheen, you don't want to use it before you tack up, but it also shines a coat and repels dirt like no other.