What vinegar can I substitute for malt or black vinegar? This is for a stir fry.

For stir fry, I usually use rice vinegar or white vinegar, or a combination of. I find rice vinegar too mild sometimes (i.e. for hot and sour dishes) so I add white vinegar.

I second the rice vinegar. I use rice vinegar in my stir frys typically. It makes them a bit tangy without being too acidic. It also mixes with soy sauces perfectly. That's how they make sauce for dumplings.

For black vinegar, I see balsamic suggested as a substitute.

It's probably too late for this answer but I would use rice vinegar, lemon juice that's slightly diluted with water, or white wine vinegar.

Depends on how much you need. If not too much, use apple cider vinegar.