How can I restore /system/bin/toolbox? I accidentally removed /system/bin/toolbox on my Android 5.1.1 tablet. Right now I have temporarily replaced it by a busybox binary I had at hand, so the system is somewhat usable but not completely functional (e.g. no network connection).

I have tried to pull the toolbox binary from other devices I have, but those are either Android 4 or Android 6, so it didn't work due to unresolved linker symbols. Is there a place where I can get a binary for a specific Android version? Or is there a place on my device where a backup is kept?
You can download the stock ROM (or any ROM, but not preferred) for your device. For most brands you'll see a system/ folder in the ZIP. Now you can take the toolbox file and send it to your device. Files taken from ROMs for other devices may work, but there's no guarantee.

If unfortunately you've found a system.img file in the ZIP, you should convert it first before attempting to extract any files from it. Tools involved are simg2img and gzip, which either comes with a Linux system or available from the Linux's default package repo (apt-get install simg2img or yum install simg2img).