I found them on ebay, they are very common in England!
Unfortunately they don't have a specific name, you can try searching for "brass mini cannon gb" Gb is the brand they have on the barrel.
Be careful because there are also larger models that are not good for playmobil cannons.
In any way let me know! Maybe you will discover better ones
P.s. You'll have to drill or bore them close to the bottom and make a touch hole to light the firecracker