Does anyone know how to get a "good" penny for these machines? Often mine are hard to interpret when they're done and just look like a garbled mess. Do I need a penny that is sufficiently old or what

The new machines I’ve used lately, my kid is into it, take credit card and have the penny preloaded

It’s probably more expensive usually $3-5 but also, it’s usually tourist trap pricing. I never have carried a penny on me since probably the 80s when I could actually use it to buy gum

I'd imagine that pennies minted before 1982 (those that are mostly copper) are best for these machines. The newer pennies (now discontinued R.I.P.) probably aren't great under pressure.

Best to use an old solid copper one, for the USA that's before 1982, for the UK before 1992.