There's this resource for starters
https://smashboards.com/threads/link-training-grounds-wii-u-edition.382047/
I believe
Stryker95
might be able to hook you up with the stage. That should help with the technical side of things.
But then your technical ability might be sufficient already, and so it depends on what level you're at. I'd suggest you take a look at the AT thread, specifically, I have linked a few guides at the very top of it in the first spoiler.
https://smashboards.com/threads/the-comprehensive-guide-to-links-ats.400585/
Read them.
The most effective way to train imo is to verse other people and focus on developing the mental side of your game, slowing things down where necessary (not just mashing buttons), really reflecting on what is happening and what you might be able to do to adapt and counter it. Eventually you'll be able to do this on the fly, but at first just take a moment in-between each game and each stock to think, and then, importantly, actually change what you are doing where necessary.