You can always try here on the Tapas forums or on the Digital Webbing forums, but what you should keep in mind is that for every artist who may be looking for a writer to work with, there are a hundred writers looking for an artist to work with. Something that takes a writer a couple weeks to write may take an artist a couple years to Illustrate as a comic.
Unless you have something to incentivize the artist into working with you, the odds of actually finding someone willing to give up so much of their time to execute your vision are pretty slim.
If you do make a post here or on Digital Webbing, just make sure you read the posting guidelines and make your story sound exciting and inviting.
Adding - In response to your second comment, if you want to formally pitch your comic script to an editor or a literary agent, you need to have a Pitch Packet put together. For a Pitch Packet, you need to have a very clear idea of your story, a concise logline, exciting character designs with visuals, and several completed sample pages. You don't need to necessarily have the artist on board that you want to be doing the final product, but you do have to have somewhere between 15-35 pages completed in a style that you feel like suits the story. So plenty of writers will hire an artist just to do the character designs and sample pages for their pitch packet, then the editor or publishing company can theoretically pair them up with an artist they have on hand.
Most publishers and editors cannot receive unsolicited submissions, which means you need a literary agent first. And to query a literary agent, you also need a pitch packet.
Literary agents will not facilitate the process of pairing up an artist and a writer for work on a webcomic, because literary agents get paid a percentage of your contract agreement, and there's no money up front with webcomics.
So if your goal is to do a webcomic, it's up to you to find an artist and convince them to work with you, either by building a relationship, or by offering money in compensation for their work.
