When you say a TV monitor, do you mean an entire TV, complete with the TV chassis with stuff like composite inputs on it? Or do you mean a TV tube that has been matched with a suitable arcade chassis?
It's going to be difficult (but not impossible) to track down a 29 inch Nanao MS8 or MS9, which is what you want for an Astro City. While they are out there, most of us hoard them like dragons hoarding their gold, as they are irreplaceable these days, and it's nice to have spares.
If you do find one, you'd likely be paying $500 minimum for it, possibly more, depending on the seller.
If the TV tube in there fits the cab well and there isn't a huge bezel gap, it might be worth investigating a universal chassis from Jomac to match the existing TV tube. You will still need someone to install it, but getting a chassis for the existing tube is likely going to be a easier than tracking down an MS8/MS9.