Midway carries PX4 springs in packs of 3. They don't have them in stock right now but are due in on April 8th which is just a few days away.
http://www.midwayusa.com/product/319...ring-3-9mm-px4
Edit: looks like these are for extending the regular mags by +3 and adding a larger base plate. Best bet for standard mag springs is straight from Beretta.