J.P.,
Did you contact the Canadian War Museum? I know in the US the Center for Military History is the actual owner of many if not most exhibits and can allocate a tank for gate guard or monument duty.
If you can't secure a Sherman through the military, you're at the mercy of the free market. Due to some resuscitated legislation, it is nigh impossible for American collectors to import Sherman tanks from outside the US. This is driving the prices of tanks available in North America upward, as the laws of supply and demand learn us.
Even before this happened, many a Canadian Sherman migrated South due to the bigger budgets of American collectors. I'm not saying it is impossible, ...
.... but why not consider a Sexton 25-pdr SP Gun? These can be had for a mere US$5,000 from SouthEastern Equipment company near Augusta, Georgia - see
http://www.secoaugusta.com > Collector's Corner or see
Joe DeMarco's pictures of March 2000.
Hope this helps,
Hanno