There are several places that have them in my area including an actual business that sells them along with pinball machines, pool tables, and other businessy pieces of entertainment. However, they are super expensive and you'll be paying over ebay for anything.
Other than that there's craigslist where cabs get posted fairly often. Several friends of mine have scored pretty big off craigslist ads so it's worth checking regularly.
The place that is absolutely tops for getting arcade cabs are arcade auctions. They are few and far between, but excellent buying opportunities when they appear. There's a company called Super Auctions that does them, but due to COVID who knows when their next one will be. I went to one of their auctions about 5-years ago and my girlfriend made me swear not to but anything no matter how cheap since we didn't have the room (and still don't). There was a Virtua Fighter 2 cab that I wanted so badly that literally sold for $25. I think I was depressed about that one for a while.
