You're looking at about $400 from a surplus store, but they'll likely be old/damaged. However, it's likely that the stores that obtained it are now illegally possessing the helmets.
Legality behind the helmet?
The CG634 Helmet is CTAT/Controlled Goods, their production, issue, return, and disposal/destruction is regulated. It's to avoid our capabilities from being defeated but also to protect the public; criminals don't need more kevlar available to them. You won't find them being sold as surplus on GCSurplus for a reason.
Helmets are kevlar and considered body armour, in many Canadian provinces there is some form of Body Armour Control Act, unless you meet the requirements of the act, possessing it is also illegal.