Are you looking for a daily carry case or just a discreet bag to transport to and from home to car and such?
Tennis racquet bags - there are cheap ones and then there are nicer padded ones with multiple compartments/straps to make them a backpack/etc...
Musical instrument cases
Yoga mat bags - some are padded and have compartments inside and outside
Soft side carrying cases for various marketing displays and art supplies/easels
Soft side, padded cases for tripods and camera/studio gear
For smaller SBR's (Uzi style), there's all kinds of generic padded laptop sleeves that should work pretty nicely. I have one that has a pouch on the outside for power supply/mouse, that works great to hold a few extra mags.
I have found a lot of the above at garage sales and such for as little as a few dollars each, most of the time in new or near new condition.
For me, I don't really care so much about the type of case from house to car, but if I need to carry something around, I have found that I need to have a reply prepared for the inevitable idiot that wants to ask about the item I am carrying and start talking about music/photography/sports/etc... which kind of blows the whole discreet thing.