Do you need the endcap (to fit a different size frame on the trainer) or the actual freehub (the piece that the cassette is mounted to)?
Here’s what an endcap looks like:
Here’s a freehub:
On the off chance you mean the cassette lockring, here’s what that looks like:
If you’re using a Campy cassette, you can use this freehub: Elite Adapter Campagnolo For Drivo/Kura, Silver | Bikeinn
I wasn’t able to find other compatible freehubs for the Direto or endcaps. Your best bet is definitely to take it to a local bike shop and have them fix it for you.