A tandem wheelset will almost certainly not fit your bike. The rear spacing on your bike is likely 126mm, tandems of the era were 140 to 145mm.

eBay, FB marketplace, and if you have one your local co-op are all places to find a decent 27" wheelset.

$$ will vary a lot depending upon source, wheelset quality (hubs, rims, etc.). For example, I have a 27" wheelset with Specialized sealed bearing hubs and Ambrosio rims -- what I would regard as a fairly high-end 27" wheelset -- I'll be looking to sell those soonish for about $150, whereas we sold a set of Normandy hub Weinman rim wheels in decent ridable condition for $50 with tires at our local co-op a few weeks ago.