I'm actually in K-W half the year (because school), and we have several fields we go to.
The main one is Flag Raiders Outdoors, and games run every other week on Saturdays. Here is a link to the FB group when we post games: https://www.facebook.com/groups/646875268686497/
The other field would be Finch Field in Guelph, and they post games here on ASC and their FB whenever they are running a game.
There are private fields but I don't know enough of them to tell you.
Check out the local KW airsoft store Blackblitz Airsoft here: http://blackblitzairsoft.com/Shop/
as well as their facebook page. On their FB page, they can help you out with rentals and questions.
Keep in mind that Flag Raiders requires PAINTBALL rated eye-pro, NOT airsoft or milspec'd eye-pro. So get a pair of paintball goggles if you want to play there (I know, I know, YUCK, but rules are rules).
Cheers, and hope to see you on the field sometime!