Magazines - get the Mec-Gar 20 rounders, they don't need the plastic to keep from overinsertion like the more expensive Beretta's, and they're just as nice. $18 from Midway with your C&R discount.
As for accessories, Houts enterprises has everything for the Storm. Dunno if he's the cheapest, but his shipping is lightening fast, great service.
For a cheap (but very good scope), I highly recommend Bushnell's Trophy 4-reticle sight. It's got a dot, big dot, crosshair and circle-dot. 30mm tube, cowitnesses perfectly with std. rings, and it holds its zero great. Very rugged and "eotech-like" to shoot. Again, Midway with the discount - $59 IIRC.
I like the Tango Down VSG grip with the Beretta forward "U" Rail.
As for other accessories, I use a std. rail-sling adapter ($8 from SG) and a 3-point sling (home made via the legendary cheap tac-sling thread). Works great.
Here's mine with all the bells/whistles:
My barrel has been shortened to 14.875", a 3-prong hider on it, but other than that it's as I listed accessory wise. I have $700 in mine, with all the extras.