Santi12008 , If the back or the battery door are broken where it hinges then you may have to replace one or both pieces depending what is broken. I will post a link below to show how to disassemble back/battery door if needed to aide in replacement and a couple links to possible parts which you can also do a search for.

Good luck. I hope this helped you out, if so let me know by pressing the helpful button.

Nintendo Game Boy Advance SP Back Cover Replacement

http://nintendorepairhut.com/repairsuppl...

http://www.ebay.com/sch/i.html?_from=R40...