Apple has just recently transitioned to an entirely new CPU architecture and they did that because the old architecture was starting to get pretty shit especially for gaming.
So unfortunately I don’t recommend buying anything too old. You want an “M1” as a bare minimum and preferably with 16GB of RAM. It probably won’t be as cheap as you were hoping, since the M1 is a fairly recent model. Anything older is a waste of money in my opinion.
Apple has two laptop lines, “Pro” and “Air”. Both will run WOW perfectly fine (if they have an M1 or newer). The Air is going to be cheaper unless you find a really good deal on a secondhand Pro.
As for where to find them - I’d start with Facebook Marketplace.
There are anti-theft features built into all Apple products. The seller will need to unlock the laptop before selling it to you. DO NOT buy anything from Apple that hasn’t been unlocked. Aside from potentially being stolen, the lock cannot be bypassed. You’ll end up with a computer you cannot use.
The seller needs to follow these steps: support.apple.com/en-au/HT201065 and as a buyer you need to make sure they have been followed.