You can find an alchemist to make it for you, or make it yourself using the apothecary skill. (That's one of those intermediate skills they introduced to allow people to make their own subcomponents.) Of course, you can always check the broker to see if any is for sale.
Since I hit 20+ tailoring I have found it is just as easy to make the tempers, washes, resins, etc myself. The stuff doesn't cost any more fuel to make, and my alt skill is nearly maxed so I make them just about as fast as a alchemist can. And of course it is still tradeskill exp :)