There are two main sizes of BB's for airsoft
6mm and 8mm.
6mm is the most common size, availiable in many different weights. Tyson did an excellent explainiation of the different weights.
8mm are used in specialty guns, (ie. some revolvers, shotguns etc). 8mm guns are fairly rare, but do exist.
There are many places that sell airsoft, and as Tyson mentioned above, Age Verification is the best way to find them.
Age verification allows you access to the classifieds and retailers on the boards. We have a strict 18+ rule on ASC. Players and retailers will not sell to you unless you are over 18. The AV Status is your proof.
Once your AVed Youll find that doors will magically open for you in the sport, but without AV, youll find it very difficult to obtain anything of real quality or value.
Do not attempt to purchase guns or receivers from outside of Canada. It is illegal and will get you in serious federal trouble. Theres a search button on the top toolbar, it will be your friend for the next month. Seriously... Anything and everything has been discussed on this board, and you will find many international airsofters come here as the wealth of information is phenominal. You just have to know where and how to look.
Good luck, hope to see you on the field.
Certified Sniper Clinic Instructor and Counter Sniper
Sentinel Arms Customs - Specializing In Unique, One of a kind guns