Depends on how bad a mismatch and how much RF you're
going to cram through it. For easier setups, like a dipole and
100w, A LDG Z100 will work. Need more? Get an LDG AT200Pro
or 100 pro. If you are QRP, try a Z11, also from LDG.

I've had experience with all the above tuners (except the 200 which
UPS left on my porch then got stolen.) Anyways, LDG makes
a fair to excellent product line HOWEVER, My K2's internal tuner
will tune a worse match than any of the listed LDG tuners. Of course
if you want a KAT2 tuner, You'll need a K2 to install it in....

I know - you were just looking for an excuse to build a K2, right?

Ken KG0WX