I assume you could always buy the tail extensions of the dominant colour in your horses tail & then buy some cheap hair dye to make it match better.
(ie: say you have a chestnut horse w/ a chestnut tail with just a little black in it-you could buy chestnut colored tail extensions & black hair dye & add it into the extensions so it matches better.) Posted via Mobile Device
There is a huge qh show barn that takes the time to glue individual hairs onto the shorter hairs to create a nice long full tail. At least they don't fall off in a lump and have a hind foot drag it around the arena.