how do you select a center channel to match when the company only makes monitor speakers?

If your HT room will have an AT screen, then a third Harbeth of the same model will be your best choice.  Otherwise, go with the best sounding speaker that will fit your space and be as timbre matched as possible.

Bill

+1 georgelofi ! Harbeth is only sold in pairs. You will never find a single one for sale.

You can set up a 6.1 system that is not much different than a 7.1 system by adding four more Harbeth speakers, or use 7 different brand name of speakers for HT.

I have a pair of harbeth supper plus speakers ...

Are those for dinner music? :D

Seriously, how about a pair of P3ESRs for the center channel? They'd share your other Harbeth's tweeter and driver types, general dispersion pattern, cabinet design, and crossover philosophy. Those are probably the most important factors in creating a uniform soundfield.

I have Coincident speakers and had the same issue.  I ran ads and looked far and wide for a single.  Luckily, a guy from Toronto had the same needs.  We split a pair of Triumphs.  Try running ads to see if someone is interested.  You may have to be the one to buy the pair but hopefully you won't be alone.  It's worth it.