Welcome to airsoft.
Like ceazer said, there's not many outdoor games in the winter that are close to Mississauga. You're going to have to drive an hour or two.
Head over to the "Players and Events" -> "Game Events and Schedules" -> "Ontario" (or click here:
http://airsoftcanada.com/forumdisplay.php?f=68) to see if people post up games where they've rented a field or something. Indoor and outdoor games will be "announced' there.
If you want a "walk in" airsoft field, the only one nearby is Ultimate Airsoft (
www.ultimateairsoft.ca) in Toronto, near Downsview Park.
As for importing airsoft guns from the states, all they need to be able to do, is shoot a .2g BB between 366FPS and 500FPS. No orange tip is required.
Now, proving that FPS to customs is on you. Sometimes they'll accept paperwork from the manufacturer or retailer, and sometimes they won't. If they don't, then it gets passed on for testing, and there is no ETA on when that will be done. The last story I heard about it, it only took 2 weeks. The common story is that it takes several months, to a year (or longer, if you're unlucky).
If your gun gets seized, the retailer isn't going to give you back your money.
The rule of thumb is to buy in Canada. It's usually faster, and less hassle/worry. Most of all, it supports the retailers that take the time and effort to get airsoft stuff into Canada for you.