Good old fashioned calling someone up, sparking a conversation and finding out their motivation/pain points is definitely more effective than mailers. It's also more cost-effective, if you have a good lead screening program in place. If you're going with direct mailings, you'll need to shell out for single page flat mailers. Those large, flat envelopes used for business purposes get the most engagement. Everyone is doing postcards, so you won't stand out, and regular folded letters just get tossed in the junk pile by default.
Of course, this is all a moot point since you're operating in an incredibly specific (but high demand) niche. In many areas of the US, anything more than 5 units is considered commercial property. You're targeting that highly coveted group of 6-12 unit, independent (not a company) property owners. This is a niche that's not going to yield many leads, so you need to court each one with finesse. Roll out the red carpet.
I don't know how it works up in Canada (although I'm fascinated to learn), but if you're not using a well-connected broker, then I would skip the skip tracing. For these elusive big fish, I'd drive out to the property and talk to whoever's managing it. Try to arrange a face to face meeting with the owner.
Of course, none of this tells you how distressed/motivated the seller is. Are you researching these properties to find those with abnormally high eviction levels, involuntary liens, various government code violations or what?