For IMM 5409 Statutory Declaration of Common-Law Union, you need to sign the form in front of a Commissioner of Oaths.
I believe each Canadian province keeps their own list of commissioner of oaths, and they are generally very easy to find, like at the local city hall. Just Google "Commissioner of Oaths" for your city and you'll find a place to start.
If you are not in Canada, then I suggest you contact the Canadian consulate in your country to see what is acceptable (public notary, possibly).