Mark,
You would use the same tubes that you would use for 700c x 20 tires. E.g. http://www.bikeman.com/
Some tubes are actually marked with widths for both 700c and 27" tires. The fact that your shop did not know this is quite sad (and revealing) in a way, although even 25 years ago I had a long argument at a shop in Belgium about this.
Cheers,
Fred Rednor - Arlington, Virginia (USA)
> Speaking of clinchers, where can I find a set of tubes that
> will fit my 27x 7/8 Clements? My LBS thought I was out
> of my mind when I asked them yesterday. If you know
> of good source that stocks tubes the size of spaghetti
> noodles, please let me know