Barnes & Noble has a page devoted to me that you might check: http://Jack-L-Chalker.search-barnesandnoble.com/.
Amazon.com is kind of flaky, sometimes even surcharging my titles, so I don't recommend it as anything but a last resort, but it can get the usual suspects. Still, if all else fails, and particualrly for advance orders, click here to order Chalker books via Amazon.com.
Borders.com stocks most of my books, and is a good source for out of print titles, but it's currently made a deal with and is being run by Amazon.com, so I'm not sure in the future you'll find anything here you wouldn't find at Amazon. For Borders click here.
Out of print? Finally, if you're looking for my vast out of print titles, most are available at very reasonable prices. One of the best places to look for these is bibliofind.com. But there is an even bigger source, particularly since Bibliofind became an Amazon subsidiary: abebooks.com. Click on author search and then type in my name and you'll find about 90% of my books there including some obscure ones and really hard to find items. Check both these for OP books.