make your own rivets, I do

but I do as my leather is thicker than the leather Brooks use.
You can buy Brooks rivets, which come in up to 16.5mm diameter for the Team Professional.
Then just stick it in a drill in a clamped in a vice and file it back to get the size you want.
Wallingford's stock the Brooks rivets
How long a shank do you need? I can check it against one of the Brooks Professional rivets I have.