The Green Party of Ontario has a good PR campaign site.
The Green Party of BC supports Mixed Member PR.
Fair Vote Canada is a Canadian site in both French and English.
The Center for Voting and Democracy is a USA site that will give you everything you want to know. It studies the impact of voting systems on political representation, proportional representation, voter turnout, and the influence of money in elections.

Canadians for Direct Democracy
Democracy Watch
Mouvement pour une Démocratie Nouvelle
Constitutional Test Case Centre, University of Toronto
Center for Voting and Democracy (USA)
Proportional Representation Library (USA)
Charter 88 (UK)
Electoral Reform Society (UK)
Make Votes Count (UK)
Fair Voting BC
Ontario Vote PR Campaign