The supply of 27-inch tires is not overwhelming, but there are plenty of them available. Where I live may be unusual, but we have a couple shops where I can usually find a selection of ten or more different 27-inch tires from Continental, Specialized, Panaracer, Schwalbe, IRC, Duro, Maxxis, etc. Even Ace Hardware carries some cheap 27-inch gumwalls with the "Bell" brand on them.
Finding a cyclocross tread in 27-inch would be a greater challenge, but I'd suggest rethinking whether you really need that. There is part of my commute that often includes from 2 to 4 miles of packed dirt and shallow gravel. I'm running 27x1-1/4 Continental Ultras, and find that they handle it every bit as well as the cross tires I was using on my other bike over the winter. And they run a lot nicer on pavement.