Almost every store, especially in summer.
Hardware store like Canadian Tire , Lowe's, Wal Mart or Home Depot here in north america, some outdoor equipment store and some supermarket.
They call it "the silence killer" odorless , painless if done correctly.
Thousands of accidental deaths so thats a good method if non suicidal people don't have time to react and save themselves.
When you do it correctly, it will be a good way to go.
I cannot tell you step by step how to do it, but there is a way to do it correctly, search Google and here.