The Indy-shirt was custommade for the movies, and are therefore hard to find. Noel Howard at M.B.A. offers a replica of the shirt (as well as a lots of other Indy-stuff), but are due to close down business September 2003. If anyone else is to take over is yet unclear.
The shirt is a kaki safari shirt, with epaulets on the shoulders and two pleats running down the sides of the shirt, bellow and above the pockets. A safari shirt can be found anywhere but the hard thing is to get one with the two pleats.
Again I turned to L.L. Bean who offers a "Cotton Poplin Long-Sleeve Field Shirt", (colour Tan) that is very close to the original. And futhermore - the quality is very high! But this shirt do not have these "Indy-pleats" either.
It might be a little hard to see, but the shirt do have the pleats - they are added later. I simply ordered two shirts, and used one as "donor", and took two strips from the lower bag at the one shirt and had it sewed on the other shirt.
As You can see, it worked out quite well: