Where can I purchase extra airtime for my phone ?
Call 611 and tell them to add more minutes. It's $10 for an extra 100 minutes.
Alternatively, you will automatically be charged for overage charges for going over your minutes.
Depending on how many you use vs. your current rate plan, it may be wiser to switch plans entirely if there's a better deal out there.
Switching plans is good, but keep in mind that the in-market promotional plans are only for renewals and new activations. However, if you do visit a dealer, there are some things they can help with...