I think it was the ole standard white dickies. I have the dye info stored somewhere as to what Rit dye was being used and in what quantities if you need to get that purple blue hue.
I've got to be honest with you guys... when I was checking out the real Jango costume at Celebration II, I would SWEAR that the short sleeve on the suit was nothing more then a heavily washed out, t-shirt!
I bought a cheap pair of white coveralls dyed them to the right color and for sleeves I just pinched the fabric to make a collar around my arm, got someone to sew it and leave a gap for the hoses. That way the hoses dont make the sleeve stick out. Looks just like it does on the real suit. Total cost $30.
I just went to the Army Navy store and got and got dark blue flight suite. It was only $30. I am now making it lighter and then dropping in some purple dye.