if you go to a hydraulic shop yes they will sell you the dust seal if that's all you want.
problem is, if you see oil, it needs more than a dust seal. The dust seal is the first seal you see on the outside. Typically folks see oil, then they'd bring it to us (dealer) and we'd put a kit in. As long as the ram wasn't bent and the bore was round and not scratched up, they were rebuildable. Then they'd come pick em up and gripe about a $100 repair bill, and most of that was parts. Kubota don't sell just a dust seal or just a gland o-ring for a reason. Typically when you see oil leaking, it needs almost every seal and o-ring replaced.