You can find free-to-use portraits in the Ultimate FEGBA Asset Repo(make sure to give credit, though. No one likes an art thief). As Blademaster said, you could also make your hack with placeholder portraits(either from base FE or F2U assets), and once people see that you’re willing to put in work for a hack, rather than just asking other people to make hack, then they might be more willing to contribute stuff like portraits.
You could also make portraits yourself. There are plenty of good splicing tutorials/resources out there, such as the Spriter’s Resource(general resource with tools, links to tutorials, etc), Toaomr’s Splicing Tutorial, the Spritans Pin Dump(another resource hub), Ganzap’s Simple GBAFE Splicing Tutorial, and the FEU Discord, where you can ask for help in a more relaxed/faster response environment.