ThemePlaza: https://themeplaza.art/themes (site that allows you to scan and download custom themes through Anemone. Using the website with QR codes is greatly preferable over the built in theme browser in Anemone as it is slow)
The Emulation Wiki: https://emulation.gametechwiki.com/index.php/Main_Page (This is a great general source for emulation information, but also has great resources for emulation on the 3DS. It's a very useful site imo)
CD Romance: https://cdromance.com/ (This has quickly become one of my favorite rom sites as it has a great selection of translated games and DS roms in general. Hshop is still preferable for 3DS games however.)
Vimm's Lair: https://vimm.net/?p=vault (The tried, the true. Vimm's Lair is a good source for clean rips of official roms. Their gamecube isos are nkit.iso but that hardly matters for the subject at hand. Imo, they are the best option for GBA games.
If anyone has any questions feel free to reach out.