The Indy-bag is actually a modified gasmask bag from WWII. Its a model Mk VII, pretty common in the Commonwealth-countries, such as Canada, Great Britain and Australia.
The original webbingstrap has been removed (who knows, maybe he used it on his pants!), and a leatherstrap with a buckle is attached to the rings on the bag.
It is no problem finding these bags, but the usually stores know that this bag is a collectors item, and they are often very pricy. You should seek this bag in surplus stores in Great Britain or other commonwealth countries.
And of course, my bag is not screen acurate, it's about 40 years too new, but I love it, it's an upgrade from the one I made out of a pants leg, and it holds what I put in it! (thanks Dad!)