27" wheels were around long enough that they still pop up on CL regularly (or at least around my parts they do), so you may want to try scouring around for a few more weeks. If you have a bike co-op in town you could try there as well.
It it were me though, I would re-space the rear for 128mm and be done with it. With 128 you could fit 126 or 130 easily, and in fact some vintage bikes came spaced like this when the 130mm spec was new. This way you could search for vintage 126mm hubs/wheels or more modern 130mm hubs/wheels.
Why would you want to find a vintage 126mm wheel if you're going through all this trouble? Your rear wheel most likely is a freewheel which is incompatible with the freehub system found on most modern wheels. Most road hubs using freewheels were 126mm, vintage MTB hubs that used freewheels were 130mm so the 128mm gives you lots of options. Going to a freehub system would snowball into a lot of other changes on the bike.
It should be warned though that although re-spacing has been done by many, many, many pro and amateur bike hacks alike, there is the danger of causing damage to the frame, particularly by the rear brake bridge.