There are comic conventions in pretty much every major city.
GETTING A TABLE
Getting a table is easy, just do it early. Look under "exhibitor" on the web page for a convention. Tables can and do sell out. A table can cost anywhere from $75 to $500 and beyond, depending on the convention. The more expensive ones tend to be at the larger and more trafficked conventions. With some conventions, you now have to fill out an application and be approved.
In addition to the stuff (your comics) that you will have to sell, you may want banners, signs, as well as business cards and a takeaway or two. A takeaway being a color brochure with some art on it and contact information. Something striking that people will tend to save. This way even people who do not buy your wares will maybe snatch a drawing and save it.
That said, I think renting a table at a comic convention is wrong in the long term. It is for me.
YOUR CLIENTS ARE NOT WALKING BY THAT EXPENSIVE COMIC CON TABLE YOU RENTED
Who will be across the table from you? It's going to be fans. Fans of Batman, Dr. Who, manga, cosplayers, etc. These people are not looking to hire you. A few out of a hundred might buy your book. But that rarely even covers costs, unless you are working on a recognizable character that people are willing to spend money on.
YOU CAN IMPACT CLIENTS WITHOUT THE OVERHEAD
If you are just starting out, it's possible to make an impact without all that overhead cost. Just attending a convention where there are potential clients can be a boost. Look at a comic convention web site to see who is going to be there. There should be people in publishing, gaming and other media outlets. Most of them are editors, owners and other people in decision-making positions. Some of these people may take a moment to look at a portfolio.
GO TALK TO BUSINESS PEOPLE
Also: consider going to a trade show or trade expo instead of a comic book convention. Trade shows are usually business-to-business, and this would be an opportunity for a proactive artist/cartoonist/animator to walk around, and meet business people who are always looking for someone to provide some striking visual content. And they have corporate money. Bring takeaways.
The point of all this is that cartooning is a commercial art. And if you can promote yourself, then that can make all the difference. It's all about being persistent, pleasant and professional.