There are a variety of sources from which you may obtain LEGO®. Sometimes a call out to parents or your school council for donations of used LEGO® works well. You can also get lucky scouring thrift stores and watch ads on Kijiji, EBay, etc., but be sure all the pieces your team uses are LEGO brand and not just LEGO® compatible. There are many websites that also cater specifically to LEGO® and are wonderful if you are seeking a specific part, such as Brick Link and Brick Owl. If you shop around you can also find some great deals on sets on Amazon, but be aware of shipping costs and possible brokerage fees if purchasing from outside of Canada.
Local stores such as Walmart, Toys R Us, etc. can also be a good source for sales if you want to pick up a set or two to use for extra parts. Educational sets are only available through specific stores.
The official supplier for Ontario FIRST® LEGO® League Jr. is Spectrum Education. Sometimes discounts can be had with bundled sets and bulk purchases, so be sure to ask. Brault & Bouthillier is another Canadian LEGO® source.