Canada: If I quit my job, do I still get my sales commission?

If you mean future commissions on past sales, yes you forfeit them. If you mean the sales you have recently made and will make prior to leaving, those commissions should still be paid. Basically, there are two parts to a sales commission/bonus, the initial sale, and maintenance of the customer. If you are in, say, insurance sales, the fact that you initially signed up a customer does not allow you to receive commissions forever should they keep the insurance and pay further premiums. Since you will no longer be maintaining the account, or providing service to the customer, you will not receive any benefit from their future actions.

You are entitled to commission on everything you sold up until the time you leave the company. Keep records and copies of everything with dates, in case you have to prove it in court later.