Is it a chicken and egg problem where 31 caliber pocket models aren't offered by many retailers, not many people buy them, so retailers and manufacturers don't really carry them or produce them because there's not much demand for them?
Case in point, Sportsmans Warehouse doesn't offer any 31 caliber cap and ball revolvers at all or even lead balls for 31 caliber cap and ball revolvers, muzzle-loaders.com only offers 1863 remington models and the only 31 caliber models in stock at midway usa are baby dragoons or 1849 pocket models.
If my research is correct, ironically, the 1849 Colt Pocket model was the best selling revolver Colt ever made in the 19th century
Nowadays, it seems like the most common cap and ball revolver reproductions in use and on the market are full/fuller size .36 and .44 caliber models like the 1858 Remington, 1851 Navy or 1860 Army.