If you are looking for something specific then I would recommend kijiji. Have you tried that?

Other than that I get my stuff mostly at Walmart or Scholar's Choice/Mastermind but it does get expensive so I have to really think it over for a while about what I am going to get and whether I really need it.

To be honest I mostly get everything new because I am worried about recalls on older stuff or how clean it is. However...I am picking up some baby legos that I found on kijiji tomorrow.

Sometimes Walmart has great clearance sections in the toy sections during the week.