In the "FIND" box, type in "comic books" (without quotation marks), and place your zip code in the "LOCATION" box. After clicking on the "FIND" button you should get a list of local comic book dealers in your area. If no dealers are shown, try a neighboring zip code. There are also comic book dealers located online. I'd suggest going to a local dealer first since you can physically see and touch the comic books you're purchasing. Also, many local dealers have memberships where you can get a discount on purchased comics.
Buying Comic Books from Wholesale Auction Houses
Finding an auction house in your area is a great way to pick up back issue comics at a fraction of the cost that you'd pay a dealer or online. In fact, many dealers get their back stock from auction and warehouse sales. They're easy to find, fun, and a great way to increase your collection (or store inventory).
The web site that I use is AuctionZip (http://www.auctionzip.com/). It's very easy to use. Just search for auctions by typing in your zip code, click how many miles you want it to search from the drop-down menu, and use keywords (such as "comics" or "comic books" (without quotation marks)). Then, browse through the search results. I've purchased comic books for pennies on the dollar at nearby auction houses. Some I keep in my personal collection, others I sell online.
Buying Comic Books Online
If buying comic books online is your choice, then just search for comic book stores online. There are several ways to do this. However, please note that there can be some risk involved when buying comics online, or contacting people via the Internet.
The easiest way would be to search "buy comic books" (again, without quotation marks) from Google (http://www.google.com). Then, click on any of the site suggestions from Google.
Craigslist (http://www.craigslist.org) is another easy way to find people who are selling comic books. Once on the web site, just navigate to your location by choosing your state, then city (for example, San Diego, California). Then, search for "comic books" or "comic collection" (without quotation marks). the downside here is that you might have to physically meet with people in order to purchase comics, however sometimes sellers will ship comics to you. The best deals I've ever found on whole comic book collections has been by using Craigslist.
Ebay (http://www.ebay.com) is also an option for online purchases. The only caveat there is that you have to set up a user account to make purchases and all purchases have to be made through Paypal, which can sometimes be a mixed experience for people. eBay does tend to be the best location to pick up back issues for comic books though.
You can also search eBay Stores (http://stores.ebay.com/) for comic books. These are online stores that are run by online comic dealers/sellers or entrepreneurs who are trying to sell their excess comics.
ComicSeeker - Online Comic Book Search
ComicSeeker.com (http://www.comicseeker.com/) does a nice job of searching several online databases for comic books. It's search results range from items found on eBay to Heritage Auctions, with several hits in between of other online comic book vendors. This is a good way to see not only which comics are available online, but also the asking price for a specific comic book amongst several web sites/vendors.