What exactly is your problem? Why do you want to burn a human effigy? Are you being invaded too many times? Because that's the only reason to burn one. Or do you mean that you want to reverse hollowing so you can summon help?
The old lady merchant sells a few, although I'm betting you bought those already. Have you gone back to the three old Fire Keepers at the beginning of the game? If you talk to them, they give you six of the things (if I remember right). Otherwise, do you have any Bonfire ascetic? If so, find a place where you obtained one from a chest (you might have to look on the internet to find an area), and then burn the ascetic at the nearest bonfire. That causes everything to respawn. However, by doing this, it will also respawn all enemies, the boss included if you're near the boss, at a higher level, so be prepared for that. And it cannot be reversed.
You can also put your own summon sign down near a boss door and wait. If you fulfill your duty, you have a chance of having your Humanity restored. Now I'm completely guessing, but it seems to be based on how many times you died before you completed the mission. For instance, if you've died twice, then it seems you need to fulfill your duty two or three times before you get your Humanity restored. But that's just a total guess on my part.