Besides the ads on here and a couple of other boards, Leo List is you only other option. Unfortunately it has become flooded with fakes, bait and switch, and poor providers with a few honest, reliable ladies sprinkled into the ads.
The best advice I could give you when visiting Winnipeg is do your research before contacting anyone. Start by reading this site and the reviews posted by reliable members.
Once you have a few names from these reviews or even ads you may have perused, search for reviews here, solicit advice from reliable Winnipeg members and use an image search function on the photos. If you let your little head do the thinking you have a very good chance of getting ripped off/disappointed.
As for agencies in Winnipeg there are no reliable good ones. AVOID! AVOID! AVOID!
Cheers
J