bavauto.com its an american website but the service is topshelf and shipping isnt all that bad. Just buy $150 worth of stuff at once and you have free shipping...then send it to Letterlock or some other postal place like that across the border so you dont need to worry about customs and handeling fees.
seeing how its a cabin air filter...... I think canadian tire (or lordco cant remember) has them for $25...lol, i mean..... im more concerned on the air my engine breathes than what I do
edit: I just realized you were asking for the 5-series.... the $25 is for the 3. I think my 528 was somewhere around $35 from Lordco.