Nobody's made one so far... Hasbro's not for certain and to the best of my knowledge none of the weapons sources (either ones that make new weapons and cast them professionally or the ones that cast other people's work in resin) have made them. MG-15's exist in the scale but they're parts of larger vehicles and so, unless you're into blowing $50 for one, they're not very practical.
They're also not accurate to the look of the Sandtrooper's weapon as well, as they're not modified at all... They're tail-gunner guns on Stuka Dive-bombers... I have one actually out of a damaged Stuka I bought for cheap one year but never did anything with it... Biding my time I guess till, well, I have time to actually use it for a good base.
Making one isn't overly hard, it's the ammo drums on the bottom that throw people off. You can get similar ammo drums from Marauder Inc. I believe (for another gun), and add them to a modified Commtech Stormtrooper rifle, which is essentially the base... It'd take some work.
The clearest shot of a stormie carrying one is the Falcon's departure from Mos Eisley. One Trooper that turns to watch the ship blast off is carrying one.