There are some around, but they are rare and command quite a premium. Original wheels are actually more common to find. I bought a boat howitzer a few years ago (that was by the way on its original carriage), and in the same place the guy had 21 original wheels lines up along the wall. I tipped off a friend of mine who got several of them. I think the late Charlie Smithgall got the rest.
The carriage on my boat howitzer was metal, but the wheels were wooden and original. They needed some TLC to tighten them up which Tom Bailey of Historical Ordnance Works did in a master craftsman-like and minimally invasive way.
Also a few collectors like to get carriages built with original irons. These too command a significant premium, but are easier to get than ones with original wood.
