If you want something not on oekaki there is a thread here for that;
Forum/viewtopic.php?f=120&t=3000397For on the oekaki this board is probably best;
Forum/viewforum.php?f=113That board is mainly things like 'characters for character' (C4C) which is designing character for each other, or mine vs yours which is like a 'draw this character in your style' type thing. Things which don't need any kind of mutual agreement on what each person will draw first, so you might get more interest with those types of things but art trades should also be fine there as well.