The one I use is called gpsp, make sure not to confuse with gpsp kai,
Its currently one of the best PSP-gba emulators around.
Google for the download. Sorry, cannot upload links yet :(
Google gpsp v 0.9 qj (qj is where I download 90% of my psp stuff from.)
Unzip it.
To use it, you must be running Custom Firmware.(In some cases, you must have the 1.50 kernel add-on, depending on what cfw you have.)
Download it, and place both the gpsp% and gpsp folders in the GAME150 folder, or if running on 1.50fw in GAME folder.
If running Sony Official Firmware, You must downgrade to 1.50fw and upgrade to cfw (uprading to cfw is optional.)
It will not work on any OFW PSPs.
If your psp firmware is higher than 3.50 ofw, then you must use pandora's battery and a magic memory stick, which is a whole other story. If you need help with pandora's battery, google or youtube a guide. If you need any further help, just ask!
~T