Apparently K-Mac make one...$420...22 mm. I got that info from Kevin at K-Mac. I got a 23 mm one from a GT so if somebody is wrecking GT that’s an option. ipd has a close-out on some rear 240 bars...shipping is a bitch though:
https://www.ipdusa.com/products/21840/100963-volvo-closeout-rear-anti-sway-bar-kit-240-260-1-0-version-1-aftermarket-100963?vsrefdom=adwords&gclid=EAIaIQobChMIgMr2oOY5AIV0YBwCh1cGw7LEAQYASABEgJ7GfD_BwE
Ultra Racing also make one but only 20 mm.
http://ultraracing.my/ecatalog/index.php/product/volvo-240-rear-anti-roll-bar-rear-sway-bar-rear-stabilizer-bar/
Another option is to make a couple simple plates to allow mounting a 2nd bar on. Google double rear sway bar Volvo 240.
I’m trying to find a larger front bar to match my 23 mm rear bar...have a Falcon one I’m going to try.