There isn't really that much of a choice when it comes to buying pure white, modest and figure-concealing 'dresses' for the pilgrimage to Makkah on Islamic websites. Of course they are available but I am yet to find any that look good in quality and come with a modest price tag too..
I have been to Umrah several times (AlhamduliAllah) over the past few years and generally I have worn a 2 piece, white prayer set consisting of a long Khimar and skirt. It was okay but I had to wear trousers and a long sleeved top underneath and sometimes this got a bit much in the hot weather.
After looking on the Internet, I found these white 'temple dresses' used by Christians and thought they would be perfect for pilgrimage. They look like they are good quality and with the average price being around 40 dollars (20 pounds) I think they are a bargain. I think they would be best worn with a Khimar especially if you are top heavy or a large, light weight white Shayla that can be pulled down over the breasts for extra coverage.
Here are a few of the dresses I think would be most suitable:
Best worn with one of these..