Definitely better to buy from various craft and tail makers, which is to say you'd have to pay me to walk into a Hot Topic.
I'm with Shim on this one. Not really going to find something worthwhile at a retail store. The best place to be able to walk up and buy a tail on the spot would be at dealer tables at a convention.Fursuitsupplies.com
has some very basic tails available. Would be a good option if you're on a tight budget. Though the colors that are still in stock seem fairly limited.
Alternatively, some suitmakers do offer a small selection of pre-made parts such as tails or ears, or some might take small parts like that on commission as well. JillCostumes
has a much better looking option available, though it is a bit more expensive. You could also hunt around FA for other makers that might offer single pieces, or perhaps search some of those custom craft sites like etsy or similar to see what sort of tails are available there.