Jalabiyas (or galabiyas as the Egyptians would say :D) are dresses that are popular across the Middle East, and often worn at home when entertaining guests or at parties. These ones were featured in Hijab Fashion Magazine, from a store called Al-Haramlek.
I can't find anything quite as elaborate as these, but you can find some pretty, inexpensive cotton ones at ABA Kaftans and Al Jilbab. Islamic Gifts stocks a few fancier ones, priced at around £50. Click on each to buy: