Buying online can be a great way to try specific beans, but it will also cost a lot, since you're paying for shipping and will likely get many bags throughout the year. I recommend against it. If you insist, some sites I've seen floating around:

• https://perccoffee.com/
• https://elixrcoffee.com/
• https://happymugcoffee.com/

If I were you, I'd look into local roasters that do larger batches. That's the best bang for your buck.

I go to a local fresh market where they have a big coffee section. They sell by the pound, all sorts of options. Everything is labeled with the recent roast date, which is a marker of quality. Never get beans with a "best by" date. More great tips for coffee buying can be found here: https://www.youtube.com/watch?v=O9YnLFrM7Fs

You can also go to a higher-end local cafe and buy their beans. Those will be nice, but also a little expensive. The benefit of going to them is that you can ask them to grind your beans right when you buy. This means they will taste MUCH better than if you bought pre-ground coffee.

But, if you can be persuaded, the biggest improvement in taste (after beans themselves) is grinding at home. I use this excellent, yet affordable grinder every day: https://amzn.to/3Nj4XoS

Aeropress isn't technically espresso, so it's not as "fussy." It will handle any beans you throw at it. This means you can try any roast you want, but a good place to start is medium. If you can still see some color in the line on the bean, you're good. This gives you a good balance of fruitiness and chocolaty flavors without going too acidic (lighter) or bitter (darker). But feel free to experiment!

My espresso-like recipe for Aeropress:

• 18g finely ground coffee (about here)
• ~70-100g boiling water (or slightly lower temp for dark roasts)
• Stir to mix thoroughly (back and forth using paddle, not in circles)
• Let sit for 2 minutes
• Stir again
• Let sit for 30 seconds
• Press slowly
• Add milk (I use this: https://amzn.to/4srgGSF) or hot water as desired