wireless backup drive like the Time Capsule?

There is nothing exactly like the TC .. and since the stock has not yet sold out why not buy a new one.. it is still the cheapest option.

You can buy a NAS.. and plug it into a wireless router.. end result is a wireless backup target for Time Machine. It is fairly expensive to buy a good brand NAS.. our recommendation is Synology as they work very well with the Mac. Wireless router can be anything you like e.g... I recommend the Asus higher end models as having a better firmware than most other domestic brands. The higher end Asus also allow you to plug a USB3 hard disk into the router and supports Time Machine.. it is not as reliable as a TC but a better router and better wireless and you can buy any size hard disk you like.. Speed turns out to be about the same as internal disk of the TC.. so not bad.. the Synology as a full NAS is a lot faster.