Best Battery Replacement Gameboy GBA SP

Dear readers,

I bought an old Gameboy GBA SP and I am interested in replacing the battery, since it displays a red light almost immediately after I have fully charged it. Hence, a new battery seems to be the solution.
However, I do not know where or which replacement battery I should buy and would kindly ask if someone here knows which one to buy. Would be great if I could buy it in Europe, since thats where I currently am

Really appreciate all the replies!