Retroarch save states are saved in the same folder as the roms; same file name with the appropriate extensions. I used VBANext for a long time before using RA and continue to use the VBA folder structure for my GBA games:
For example, Metroid Fusion:
/dev_hdd0/game/VBAM90000/USRDIR/roms/Metroid Fusion (U).gba
/dev_hdd0/game/VBAM90000/USRDIR/roms/Metroid Fusion (U).state1 ...or .state2, .state3, etc.
RA handles GB games with a different core, iirc. Personally, if you are mainly trying to emulate GB, GBC and GBA games on your PS3 I'd go with VBANext instead of Retroarch as VBA can handle those specific 3 without switching cores; just my two cents.
Just to add, if you insist on using RetroArch their folder hierarchy suggests the following:
/dev_hdd0/game/SSNE10000/USRDIR/cores/roms/vba for GBA roms
/dev_hdd0/game/SSNE10000/USRDIR/cores/sram/vba for srams (traditional console "checkpoint style" saving) and
/dev_hdd0/game/SSNE10000/USRDIR/cores/savestates/vba for RTS (real time saves.)