Most new smart phones and Ipads are compatible with the 3G or 4g networks in the US and Canada & the rest of the world - my equipment works find in the US. In regards to purchasing a SIM card in another country for your equipment to get a cheaper rate, you will need to make sure that your equipment is not locked by the carrier you purchased it from - many do lock them - so you are stuck with purchasing their roaming data plans and can not just replace the SIM card with another carriers SIM card.
Also CDMA devices do not have SIM cards so purchasing a different data plan in the country you are visiting is not an option - you are stuck with using your carriers data/voice roaming packages.
In a situation where you have a locked SIM card or a CDMA phone it really is cheaper to just purchase a cheap pay and go phone in the country you are visiting and throw it away when you are finished with it,
In Canada you can purchase prepaid SIM cards through two of the large carriers - Rodgers & Telus and most probable Bell. Forget Wind Mobile. The big thing is your equipment must have an unlocked SIM card.
Telus actually has an online site where you can check your none Telus equipment to see if it is compatibly