I did a tour in Nova Scotia in '80, on 700c clincher wheels, and in the middle of the tour had to have spare 700c tires shipped from the AYH store in NYC to I-forget-where, Ecum Secum?, a week ahead of where we were, because no bike store anywhere had 700c tires in stock. The widest tires I could find were 32mm Hutchinsons, and they were absolutely the worst tire I ever rode in my life. Daily multiple flats, all my tubes had multiple patches, and my friend mercilessly mocked me for being cutting-edge and running 700c. He had few flat problems, and could get replacement 27" tires at any/every bike shop on the way.
I had the opposite experience touring in France in '74 on a bicycle purchased in Amsterdam. I was running 27s and in deepest, darkest France one could only find 700C tires. I had to take a train to a fairly large city to find the tires I needed.