In nearly all cases where you're writing a webcomic, you are going to want a true collaborator. Comics are a visual storytelling medium, as evidenced by the fact that you can have a comic that has pictures but no text, but you can't really have a comic that is text without pictures. This person is going to be helping you to tell your story and should ideally have some freedom to make contributions of their own. The artist is going to be providing a service that you can't provide yourself, so you will want to find someone you can trust to not just follow your instructions, but know what is going to work visually.
In addition to Craigslist and Deviant Art, there are some sites and forums around that help writers and artists find people to collaborate with. A search for something like "comics artist collaborate" should turn up some possibilities. The person who draws the comic is usually just referred to as the artist; I don't see "illustrator" used as much in this context.
I would suggest having between a week's and a month's worth of strips written before you seek out your collaborator. Aside from helping to show a potential artist that you're serious, it's just a good idea to have a lot of material ready in advance, just in case something comes up that prevents you from writing new comics for a bit. I don't know if you need to do complete character designs, but you will probably want to write short descriptions of all of the main characters and settings so the artist knows what he or she will be expected to draw.
