The Indy-bag is actually a modificied gasmask-bag from WWII. Its a model "Mark VII (seven)", that is pretty common is the "Commonwealth-countries", as Canada, Great Britain and Australia.
The original webbingstrap has been removed, and a leatherstrap with a buchle is attached to the rings on the bag.
It is no problem finding these bags, but the usually sourches knows that this bag is a collectors item, and they are often very pricy. So unless You are comfortable in paying "through the nose", I advise You to seek this bag in surplus-stores that is found in Great Britain or other commenwealth-countries.
I got my bags (3 of them) in Australia, and the price was very reasonable...
My bag on display