What I’ve been doing with mine is posting them in the Community Chat channel. 
I used to also do it on here, but figured everybody can see it if I put it there. Some who offer them plan in advance but I’ll usually just give a 10 min warning (often will wait a few minutes longer than that though as I know there is a chat delay sometimes). But, if I see anybody asking in chat and I’m available, I’ll sometimes go do one as soon as I reach a good stopping point with what I’m doing.
Illuminaughty has also started a service for them. 
They’ve got a Discord (Exo Guild has one too but I don’t think it is being used) -