So let's talk materials in general...
For the gown, I've been debating between cotton voile or cotton lawn. Both are thin, semi-transparent, and gauzy. In my last encounter with voile, however, I found it to be THE DEVIL. It sticks together and is generally difficult to work with. Lawn is much easier to work with, though less floaty, which I think may be essential to this dress. Both of these fabrics require petticoat(s), as going without will show the world a bit too much. The solution to the problem is to use both! The top layer of the gown will be voile, the interlining lawn, and the lining layer a slim sheath of muslin (the petticoat). Three layers may sound like a lot, but the weight of these fabrics is so tissue-thin that it will not poof out the way any heavier fabric would.
Here are some sources I've found for voile and other such lovelies:
Dharma Trading Co. - white 100% silky cotton voile (that's silky, not actual silk).
Prices range on all of these fabrics, but none are all that high, which is encouraging, since there might be quite a lot of yardage in this gown.
Here are two linens I found that looked suitable. Unlike the voile and lawn fabrics, linen is widely available in just about every color, and many blends:
Dharma Trading Co. - white 55/45 Linen Rayon blend (to be dyed yellow or any color)
The robe may or may not need to be lined, depending on the weight of the linen. The idea is for it to be floaty, summery, and cool (temperature-wise).
These fabrics will need to be ordered (boo), but once they arrive, and all other costume nonesense has been completed, I will begin the gown!