You can watch the auctions, but it’s uncommon, and you’ll still have to pay if you find any.
Occasionally on Gu-nP-ost shells turn up:
https://www.#######.ca/ammunition/live-ammo/markham-york-region/gambore-regal-diamond-25
Or retailers, but here may be shipping to consider:
https://www.triggersandbows.com/Shotgun-Shells-c156844545?attribute_Brand=Kent
https://www.reliablegun.com/shotgun-ammunition#/specFilters=3m!#-!32
These are all just hunting loads.
Target loads ain’t happening’.
Load your own.
Or if you don’t, as I don’t, there’s a Federal 12g x 2 3/4” 7/8 oz #8 at around 1200 fps that I really like.
If you can find flats of these they work well for some shorter chambered guns that I know have strong barrels.
Price is normal too.
Primers stick on opening sometimes, is the only thing I’ve noticed with these.
The trick of lifting the gun up and letting the forend fall onto the palm on opening works fine for me.
Anyways a nice light load worth trying.
Btw, some classic guns are meant for a hundred rounds on the clays course!!!