Float a Boat has an extensive range of paddle wheel kits made in plastic. They don't have them listed on their website but they're shown in their newsletters, or asked them.
https://www.floataboat.com.auClyde Model Boats used to make brass kits. I'm not sure if they're still going. Talisman on this forum was behind it you could ask him.
There were some German, I think, kits made in brass. I don't have the link for them any more. But sit down when you look at the price.
What boat were you making?