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

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.