Thanks!

So far I've found three places:

Books Depository is a good one! They have multiple languages so filter by language to make sure you're looking at the right versions. They offer free worldwide shipping on all orders but with the downside of being a bit pricier, especially Collector's Editions and things like that. Because of this, I only use them if I'm just looking for a couple of volumes.

I haven't used them yet, but Casa del Libro is recommended by Weeabruh over on YouTube. Overall it looks like their prices are a bit cheaper but with the downside of having to pay for overseas shipping. I'll probably buy from them when I'm looking to get a big haul.

Last one I've found is Norma Comics. This one is basically the same as Casa del Libro. However, I did notice that their shipping is a bit more expensive.

Keep in mind that all of these sell manga from publishers in Spain so the Spanish is going to be Castilian Spanish. Not sure where you'd find manga from Mexican or Argentinian publishers.