OK, so here's a constructive suggestion on a possible way to find old catamarans.
When there are catamarans for sale in the paper, call them up & make a pitch to them for donating to your organization. As a previous poster recommended, if you have tax-exempt status, donations to your organizations may be tax-deductible, this may actually be better for a seller than getting a pittance for it. Now, regular newspaper classifieds cost $$, don't run for long. Some people who have an old catamaran in the backyard may not want to spend much $$ to advertise & get rid of it.
Also, our newspaper has an online website, and they have online classisfieds that include all the ones in the paper, but also include a LOT more than they print. Right now, there's 2 cheap boats listed online, that aren't in the paper version. An 18' cat for $650, and a Sunfish for $450, both with trailers.
When I was looking for a beater Sunfish (i.e. old & cheap) one time, I found good luck in the cheapo classifieds. In my section of TX there is one called "The Greensheet" and another called "The Thrifty Nickel". They are extremely cheap to advertise in (sometimes they are actually free to advertise) I'm sure there is some kind of thing like that in your state. I scanned those publications weekly until I found one. Perhaps these kind of publiciations in your area will occasionally carry an ad for a catamaran. Maybe the kids can be assigned to monitor the publication, like picking up a copy every week at the supermarket, scan it & call the ad. These kind of publications, generally have lower cost items in them, since it's cheaper to run the ad - lots of flea markets, garage sales, fix-it guys and shade tree mechanics advertise. So do people with something in their backyard they want to get rid of without spending $$ to do it.
Sooner or later, you might hit paydirt! Maybe these publications even have on-line classifieds. This may enable a wider search range. The supermarket may only have the local edition, but on-line, you could find one from a city an hour drive away. 2 hour round trip for a free boat might be worth it?
Example comparison - local newspaper classified is full of motorboats $7K-$30K, and sailboats $5K-$40K. Greensheet has lots of 14 ft jonboats for $300-$1500, a 22' keelboat for $2000 and a catamaran for $650.
Good Luck!
Also, "Auto Trader" has a related publication called "Boat Trader". Sometimes folks advertise cats in there. But, cost to run ad is actually more than newspaper, so this may not be as good.