I actually had this conversion done a few years ago; parts are readily available, and where they are offshore, bringing them stateside is not an issue. My project (I also had the manual trigger converted to automatic) involved a favorite pet 626 I'd bought new at Beretta Gallery in 1996, so I didn't mind lavishing some money on it. Parts required are below, as well as sources; numbers correspond to Beretta's schematic:
Single trigger to double trigger:
35 Non-selective safety
64 DT spacer
65 DT connector/stock bolt block
66 DT screw
67 DT plate
68 DT screw
69 Left trigger
70 DT screw
71 DT articulated plate
72 Articulated trigger
73 DT trigger guard
76 DT pin
77 DT spring
78 DT pin
Manual safety to automatic safety:
38 Safety spring
39 Screw
88 Auto safety cartridge latch
89 Auto safety washer

The 626 has a reputation for reliability, but should replacement parts ever be needed, there are several good sources:

http://www.omps2.it/en/firearm-spare-par...-cal-ga-12.html

http://www.brignoliarmi.com/web_eng/shop2.asp?id=6&id_1=3

http://www.midwestgunworks.com/page/mgwi/ctgy/beretta-625-626-627-parts

https://www.gunpartscorp.com/Categories/Beretta-33152/Shotguns-35390/626-43018.htm?page=2