I buy 'em at local bike shops in my area. 24" tires are common on kid-sized mountain bikes (for 9-12 yo'd). Most of the major bike manufacturers make those types of bikes, so any dealer should have some or be able to easily order you some. I know Trek dealers sell them (the Bontrager brand), so you might want to try one of those if any in your area. Might be a little costlier than Amazon.com, but who wouldn't pay a little more for mom, right? Mom will also know where to go if she needs bike repairs or wants to buy some bike stuff in the future.
Also check BMX bike shops. There are a few BMX bikes with 24" wheels, though not as common as the 20" BMX's.