Do I need to obtain a visa before arriving in Thailand or can I extend my stay after the 30-day exemption?

Also it is not a 30 day visa on arrival it is 30 days visa exemption

You can apply up to 3 months before travel for an evisa. TR60. Gives you 60 days when you arrive which you can extend for 30 more at any Thai immigration office

you either apply for a single entry tourist visa from the eVisa website at the thai consulate in Ottawa or Vancouver <- which will give you 60 days when you arrive and which you can extend one time for 30 days once you're here giving you 90 days

OR

You show up here without a visa and get stamped in free for 30 days on what's called a visa exempt entry which you can extend one time for 30 more days for 1900baht <-this way only gives you 60 days of stay then you have to exit/re-enter to get a new free 30 day stamp