Hey, congrats on the baby!
There are lots of places to register, but Babies R Us is the most popular and most accessible for most people to buy things for you. The nearest one is at Winston Churchill and Argentia in Mississauga, right by the Lisgar GO station.
The new baby store Snuggle Bugz here in Milton also has a registry (more product choice if you do it online
www.snugglebugz.ca), but would probably only really work if most people coming to your shower are from Milton area or are comfortable doing online purchases.
I think Sears and The Bay have baby registries, but in my opinion, both stores suck for quantity and choice of products, not worth it.
Good luck.