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

+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.

I definitely would NOT use another brand of center speaker. At best the overall sound from the front would be mediocre or it may even ruin the sound from the Harbeths. Take a look at this article,

http://www.soundandvision.com/content/matching-front-and-surround-speakers

And a mismatch between front L&R and center is more degrading than front and rear.

An absolute perfect matching center speaker would be a single Harbeth Super Plus, or any other Harbeth should be good. As suggested, try to find a single Harbeth. If this is not possible, here are some other options using 6 speakers.

3 front (L/C/R) and 3 rear (L/C/R)

3 front (L/C/R), 2 sides, and 1 rear

2 front (phantom center), 2 sides, and 2 rear

Phantom center can work well, but only when listening positions are on the center axis or fairly close. Extreme off axis listening positions from the center will not be good.