Pretty much anything that can run Linux and has a network port would work. Most languages, libraries and well known software has been ported to work the same way on a lot of hardware. Save for low level hardware specific code, the rest is more or less standard.

The Orange Pi other users suggested is a good choice, as would be the very similar NanoPi, and many others. However, although the scarcity hits more the popular boards such as the Raspberries, others also became somewhat harder to find, so I would suggest for example to take note of which board is supported by a well known distribution, then look for vendors offering it. A good start would be the Armbian and DietPI download sections in which you can easily find dozens of boards and devices with their support status.

https://www.armbian.com/download/?device_support=Supported

https://dietpi.com/#download

1.5 years ago I bought a Orange Pi Zero for less than 20 USD. I am using this device for pihole. Without any issue, it is running smoothly.

search for 'orange pi zero LTS', shipping included is less than $30 now, I got it for $18 all included before the pandemic(a case, power adaptor). it's solid to run pihole for me.