AFAIK MTH does not typically sell their tenders separate from the locos. However, I've seen some dealers offer "break-up" sets where they sell them separately.
I have a couple MTH loco and caboose sets. While the set has one number, sometimes the individual boxes inside the set box have an "a" or "b" affixed to the product number - which makes me think they may be available separately - but probably not directly from MTH.