It's not the prettiest look but I managed to get a 1600 mah rechargeable battery pack in my GBA 101 : r/Gameboy
Amazon.com: Game Boy Advance SP Replacement Battery Pack for GBA SP (packaging may vary) : Video Games
New Rechargeable Battery for Nintendo Game Boy Advance SP + Screwdriver 850mAh – I.E.T. INDUSTRIAL ANTONIO PRIETO – SINCELEJO, SUCRE
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Amazon.com: Intec Power Station Rechargeable Battery Pak For Game Boy Advance : Video Games
Amazon.com: Xahpower Gameboy Advance SP Games Accessories Kit, 1 Pack Charger and 1 Pack Battery Pack for Game Boy Advance SP Systems(GBA SP) : Video Games
Game Boy Advance Battery Pack Review - IGN
Gameboy Advance Rechargeable Battery Mod 1500mAh Li-ion USB-C for Game Boy GBA | eBay
Game Boy Advance DIY £14 Rechargeable USB Battery MOD : r/Gameboy
Amazon.com: OSTENT 850mAh Rechargeable Lithium-ion Battery + Tool Pack Kit for Nintendo GBA SP Game Boy Advance SP : Video Games
Amazon.com: Xahpower Gameboy Advance SP Games Accessories Kit, 1 Pack Charger Cable and 1 Pack Battery Pack for Game Boy Advance SP Systems(GBA SP) : Video Games
PowerUp Advance GBA Rechargeable Battery Kit | RetroRGB
Purple Gameboy Advance InterAct Power Pak Rechargeable Battery 15 Hour Charge 741948247088 | eBay