I'm in the same position you are. I have no sewing talent whatsoever. There are a couple of options.
One possibility is to create a costume that doesn't need much 'sewing'. For example, I created a Mario costume for a friend by purchasing a red turtleneck, denim overalls and white gloves. We painted a mustache on his face and bought him a Mario hat (available at many stores). You can make many costumes by 'assembling' the individual pieces in this manner.
Another possibility, if you want a costume that has 'unique' clothing involved, is to try Etsy. You'd be amazed how many great tailors/seamstresses there are there who will often custom make garments for you. I've used Etsy a few times and always been happy with my purchases. However make sure to read a sellers reviews/check their ratings before buying. I got my son a Ben 10 costume this way. I purchased a black t-shirt and blue jeans off the rack, and he already had a toy omnitrix. I got a great jacket custom made for him on Etsy. I know I've seen people on EBay selling them too.
Another possibility is a site like cosplaysky. They have a great selection of custom costumes, including a lot of Star Wars costumes. Since there is an entire Star Wars day (Friday, I think?) this is a great source for costumes.
I've been wearing costumes to Comic Con for a few years now. My advice is to start with something 'small' and see how you like it as you wear it around the Con. Then you can tweak it/add to it next year.
HTH!