Harris/Sheldon does have 27" FG wheels, they stock one good economy model for $100 but you could get them or anyone else to build them for you.
http://sheldonbrown.com/harris/wheels1.html#630
However, most people with 27" wheel bikes just opt for switching to 700c. Cheapest wheelsets are $150 shipped, but you need all new rubber, so staying 27 can be cheaper if you just want a low cost beater. You need 4mm more brake reach (if you're into brakes), no other compatibility issues unless you have one of those forks that takes the tiny front axles, then you just need round and flat files. Some of the lower-end fujis had them, if that's what your bike is.
Edit: 700c tires and smaller volume (23 or 25 vs 1 1/4) will lower your bottom bracket 4mm plus the change in tire size, so if your bike has long cranks that could be a "compatibility issue"