You could try the 420 Squadron Association. I found these contact details for them on a website:
420 'Snowy Owl' (City of London) Squadron Association Robbie Robson, 7 Camberley Close, Hucclecote, Gloucester, England, GL3 3PQ
http://www.411.ca/ is an online phone book for Canada which you could try although it may be like looking for a needle in a haystack without some idea of which area of Canada to look in.
Another option is to try the Canadian press. If you can find out which area the men originated from in Canada you could try contacting the local newspaper in that area and asking them to run a story for you which may locate either the airmen, information about them or relatives.
Good luck & please pop back to let us know how you progress with this.