I tried for a while to find a compatible choke for my CVA 12 gauge and never found anything either. I gave up and bought another gun. But my plan when I have time to revisit the CVA is to try different plastic wads/shot cups (possibly with hand cut slits/petals) until I get the desired pattern and PoI.
Also, the modified choke the CVA came with is likely adequate for most anything you would ever use the gun for. The gun that replaced my CVA is a 10gauge Pedersoli SxS with interchangeable chokes that came with two extra full turkey chokes. After buying lots of choke tubes, I settled on a modified choke in both barrels. Loading wads through anything tighter than a modified choke gets to be a real pain.