What an excellent project. :)
I have seen these in junk shops and there are plenty still around.
How about Freecycle? This is a nationwide scheme where unwanted goods are advertised locally - you can also advertise your wants.
http://uk.freecycle.org/
Or a Want card in the local post office? If he includes a photo of one on the card, it will be eye catching. I'm sure someone has one in the shed they'd be glad to get rid of.
eBay may be a possibiility - it's worth a look.
Also, have a look in Yellow Pages for lawn mower repairers. Such a shop may have an old one they'd be willing to sell cheaply.
I have just looked on eBay - nothing suitable under 'cylinder lawn mower' but when I searched 'old push mower' this cae up:
http://tinyurl.com/2o3oxl
Too far for you, but it gives an idea.
And here are some that have sold recently, for information:
http://tinyurl.com/2vbkjn
Hope this gives you some ideas.