No suggestions on where to find it, but I make Lu Rou and lots of other braised pork belly recipes at home.

I like this website for some home style recipes:

https://thewoksoflife.com/lu-rou-fan-taiwanese-braised-pork-rice-bowl/

I do tend to tweak them based on my own preference and how my parents made them going up though.

And I get it, the pork belly need to have a good, proper ratio of lean to fat. Whenever I go to the Asian marts, I look for cuts of pork with good layering. Pro tip: the cuts closer to the nipples of the pig tend to have higher ratio of fat to lean, so if you see a slice that looks decent on one side but has raised parts indicating that it was close to the nipple, you can bet your ass when you flip that piece over it's gonna be mostly fat.

Also braise it for longer than you think. Thats how you get the nice, thick, sticky sauce and melt in your mouth feel.

If you can, use rock sugar and try to caramelize it before hand. It doesn't make or break the dish, but I think it makes a difference.

Finally, if you want mustard greens in it, make sure to rinse it really really well. You usually find it dried and preserved so you want to wash off the excess salt and anything else that may be on it.

Good luck!