I have one from Who's Sari Now that I love. It's really pretty (since it's made from a sari) and the head opening is really huge, even with hairpieces and stuff, it has plenty of room. I think they have a website, but I don't know what it is. Anyway- just a suggestion.

I bought a big caftan from Catherine's and I love it...I just pull it over my head and voila!

Ginger,
I try to find a caftan that either zips, ties, or buttons in the front so that I don't have to pull it over my head. I had to alter mine to do that. Cut it down the center add ribbon to the edges and some gold frog hooks. As far as fabric goes, the lighter the better. You wear it all night if you didn't bring a change of clothes, plus you usually put it on directly after performing. There are some heavy beautiful velvet ones with hoods but those are HOT! Mine has chiffon bell sleeves.
Some other options if you don't like the way most of the traditional styles look. I have known some dancers to use Mu-mus and/or bath robes (silky long ones). I would use your judgement on these cause some of them can be tacky!! But if you find the right one(s) they can work as cover ups. I have also known people to take a silk veil (or any material for that matter), fold it in half lengthwise, cut a hole on the fold for your head (a large one to get yourself in and out), get inside and pin or sew the 2 sides of the veil under your arms to create bell sleeves and voila a coverup!
Here are some really cute IMO (and cheap) Galabeyas on Ebay:
search.stores.ebay.com/Egyptf...QsofpZ0
You can take one of those and have someone cut it on the front and put ties or whatever in.
To sum up, it really just depends on how much work you want to put into something, basically buy something thats easy to get on and off and make sure its light!
Hope this helps!!
Joyous Dancing!!
-Nadirah Johara