According to MacRumors.com, the AirPort Extreme and AirPort Time Capsule are out of stock at many U.S. Apple retail stores. You can still buy ‘em at the online store, and the AirPort Express can still be purchased both online and in the retail stores.
With the 2016 Apple Worldwide Developer Conference about two weeks away, it’s possible that we could see updated units announced. The current models are a bit long-in-the-tooth. Apple last updated the AirPort Extreme and AirPort Time Capsule at WWDC 2013 with faster 802.11ac Wi-Fi, new designs, and other internal changes. The AirPort Express was last updated in June 2012.