Just check out you local Army Navy Store. If it's a good one, they will usually have Mechanics bags. Just make sure you get the canvas ones, not the nylon. It will last longer.
I disagree. The only GI bags I've personally seen are nylon, and they're far superior than the imported canvas type. I understand there are some older actual GI canvas bags out there but I haven't seen them. IMO stay away from the current manufacture canvas bags. I made the mistake of buying a Rothco bag about 12 years ago and it faded and fell apart with a quickness. Uncle Sam's nylon tool bags I used and personally owned GI nylon bags I use are FAR superior to the imported canvas bags.