What can one use for a replacement Time Capsule

There is no direct replacement for the Time Capsule if you mean your goal is to get another device that is a combination, wireless router, Ethernet switch, with a built-in HDD designed to be used for Time Machine backups.

However, there are some alternatives:

• A wireless router that supports connecting an external drive that could be used for Time Machine. One brand to look at is Asus.
• A separate wireless router and a Network Attached Storage (NAS) that supports Time Machine. For the latter, I would suggest taking a look at Synology.

Various of the sites I deal with use Synology NAS servers for Time Machine backups, which works well, though that also involves separate Wi-Fi hardware.

The closest direct analog to AirPort Time Capsule with Wi-Fi is a Synology RT2600ac Wi-Fi with optional attached storage; that’s one box with added (and swappable) external storage.

Time Capsule are difficult to obtain.

There are refurbished Time Capsules for sale on Ebay.com and Amazon.com.