How heavy of a team are you? 27-inch tires are getting a bit scarcer fo find now as 700c and 26" tires are pretty well in vogue now. If your LBS does not stock them some of the catalogs do, like Nashbar and Performance.
Tubes for 27" are almost as scarce but a 700c tube will do in a pinch. Any good quality tube/tire will fit the bill.
Would not overly worry about blowing tires off the rim. Have seen folks who weigh 500 lbs total ride 27x1 1/4 tires. Do inflate to the recommended tire pressure.
What kind of tandem is it that's been hanging for such a lon time?
Welcome back to riding 2-gether!