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.
[guide|12735]
http://nintendorepairhut.com/repairsupplies/gameboydsrepairitems.html
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2374313.m570.l1311.R1.TR11.TRC1.A0.H1.TRS0&_nkw=nintendo+game+boy+advance+sp&_sacat=171833