Monitor

Without knowing exactly which year/model iMac you have,

about all I can say is that pretty much any modern monitor

will work with proper cabling and or adapters.

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. Learn how to identify your iMac model.

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.

How to use target display mode

1. Make sure that your iMac is started up and meets the system requirements above.
2. Make sure that the other Mac is started up and logged in to a macOS user account. 
3. Connect the two computers using the appropriate Mini DisplayPort or Thunderbolt cable. 
4. You can use more than one iMac as a display, if each iMac is using a Thunderbolt cable to connect directly to a Thunderbolt port on the other Mac (not the other iMac). 
5. Press Command-F2 on the keyboard of the iMac. You should now see the desktop of the other Mac.
6. To exit target display mode, press Command-F2 again. Or disconnect the cable, or restart either Mac.

Here's my favorite old Mac site, you can even get a 2 year Warranty onthem...

https://www.macofalltrades.com/shop/apple-desktops/imac/21-5-inch/?_bc_fsnf=1&Year%5B%5D=2009&Year%5B%5D=2011&Year%5B%5D=2010&Year%5B%5D=2012&Year%5B%5D=2013

Beware the 2009 & 2010 2.15" iMacs do not work, & Retina iMacs do not work. :)