Well, necessity being a mother, I tried several places for "cartographic" paper - with no luck - and then found a packet of 5X7 photo protectors at a craft shop for about $5 US.
After a frustrating time of trying to apply the very limp sticky side - and either not being able to position it accurately or having unsightly bubbles and wrinkles - I experimented with the clearer, stiffer, slightly thicker, not-sticky side.
Miracle of miracles, because of the residue left from the first three attempts, this piece stuck and seems almost invisible. Now the question is, will it stay on for awhile !?!
Nina, I will call Collings shortly. $2.50 is a very good price.