Not sure what you mean by community lineart (probably due to me not being fluent in English), but you can find
lots of oekakis here, which you can use in a competition if the artist states so or you've got a permission from the artist. ;]
If you see a lineart in Competitions that you like, you can track it down by clicking on the "Based on" link right under the oekaki picture. :] Do so until you find the original piece.
You can use an editable for competition by clicking "color me in" normally, then go to save it and choose to finish it later, and then go on to Competitions-oekaki board. Open oekaki, under the drawing area are your unfinished oekaki drafts. :] Click on the one you'd like to use for a competition and post it. ^^ If you accidentally end up having that oekaki on wrong board, you can just report the topic and ask it to be moved. c:
Hope that helped a little bit. ^^;