There are two venues which host games on a regular basis. Niagara Quartermaster and DMZ Paintball. Both are great venues, however DMZ is a bit more casual, and a bit more lax with fps and other rules in general. I haven't been there in a while though, so it might have changed a bit. Personally, I prefer NQ as their staff is extremely friendly and knowledgeable. I've found the players who tend to attend to their games a bit more mature as well.
I would recommend you to Google both places, and going to the stores Both are in st Catherine's.
Also, check out their FB groups to keep up to date with events and such.
I've heard that there is a new CQB field in Niagara, but cannot confirm nor deny it.