The plague doctor avatars (there are two of them) were from the Halloween Event in either 2016 or 2017, I forget which. In any case, you can try to get them in the current event going on now.
I am not 100% sure on this, but if there's a specific one you want, I think you have to have your avatar set to a male if you want the male Halloween avatars and a female one if you want female avatars. Just keep that in mind if there's a certain version of the plague doctor (or any of the others) that you're aiming for.