I think you will find that you can obtain 20 bore 2 1/2" cartridges, in Canada through Kent Gamebore Canada.
Phone:(800) 844-1880
Fax:(905) 294-4921
E-mail: kentc@kentgamebore.com
I have found that European Arms ( http://www.euroarms.com ) in North Vancouver certainly stocks their 12 bore 2 1/2" loads, and I would think they would have 20 bore as well.
Finally, out here on the west coast, Stuart, of S J Newby Gunsmiths Ltd. (Half Moon Bay BC 604-740-3725) is an excellent double gunsmith.