I almost invariably use anime FCs, in some cases am willing to use other art (for example, for D&D campaigns) and it's not like I can't use description if I really need to. Nonetheless, in almost every scenario if an anime FC won't do, I am probably not going to be a member of that roleplay. Beyond just liking to use those FCs, they are one of the ways in which I can say "yes, me and this person share some common ground in viewpoint and expectations. I can more or less trust we are on the same page regarding what we want from the roleplay and how far the suspension of disbelief can be stretched".

I usually get them from google, and sometimes a site like zerochan.