Unfortunately you really don't have any options to buy it at a store, because it is now out of date. Adobe is actually up to CS5.5, but you should still be able to find CS5 at a good computer store that carries software. You might try a local university bookstore too. If those don't work for you, some regular bookstores may have it. But if you have to have CS4, you will probably have to buy it on line. I'd check Amazon.com. You could try eBay, but be careful to only buy from reliable sellers and that the copy you are buying is factory sealed in the box and a full, not upgrade, version, and also that it's the correct version for your computer (there are PC versions and Mac versions.)
Good luck!