Where can I find Storm Bolters
The Storm Bolter is typically a Terminator weapon; it's a double bolter. It is well illustrated on the 40K wiki. (Links below)

To make one, the easiest way is to use two bolters. Separate the hands/arm/pistolgrips from the body of the weapon. Shave off the right side of one and the left of the other, down until you've just touched the barrel. Glue these together, glue one arm back centered. Patch the gaps. Prime and paint.

To convert a terminator's storm bolter for non-terminators, cut off the weapon at the wrist, and the arm at the wrist, and swap.

Most of the terminator models I've seen ship with a storm bolter as the default weapon, and I don't recall seing the option for them for non-terminator troops since the days of Rogue Trader, but I'm a casual player only, and so don't keep close track.

http://warhammer40k.wikia.com/wiki/Terminator_Armour
http://warhammer40k.wikia.com/wiki/Storm_Bolter