Lexar 32GB MicroSD (Cheap option)
Honestly, if you use the full 128GB of space inside the Galaxy Note 9 you'll probably want to buy a card bigger than 32GB. That said, if our first few recommendations were too expensive, this is a cheaper card worth getting.
This basically gives you a little extra wiggle room without costing too much money. It's priced at $16 and still offers 95 MB/s read speeds so it's plenty fast. We've seen the price drop to just $12 in the past, so keep an eye out for discounts or comparable models. This isn't the biggest or the best, but it's good enough for most.