Nice gowns can be found at Karen Miller, BCBG, but prices are just ridiculous (when you know how much they cost at US).

Morgan de toi has lots of very nice options, between 50-150chf, and during the bi-anual sales you find things 70-90% off (but this was 2-3 months ago... and the only store open right now is the small one at balexert, the big one in the center is closed). 

Sorry not being of much help.... I'd also say try to go to annemasse or annecy, french stores are way cheaper than swiss ones. 

You should look in Chess, near the 4 seasons hotel. And across the street of the Manor there is a store called 'angels & demons' i believe, they have some long gowns as well.

Morgan has lots of nice dresses indeed, but remember that Morgan Switzerland is approx. 30% more expensive than Morgan France (as any other French brend sold in Switzerland), so you'd better go to a French Morgan shop, as the border is so close from Geneva, i.e. at Migros Etrembières near Annemasse.

The price for renting a long evening gown in Geneva could be between 100.-/200.-, probably more ... which is not cheap.

Buying one: you usually have a good choice of nice long dress at C&A, the one in Balexert or the one near Place du Molard.

The Red Cross second-hand shop, Vêt'Shop, corner rue Leschot x rue de Carouge (Plainpalais area) has second hand clothes, and also fancy costumes for rent; so they may also have evening dresses for rent ? (phone 022 321 83 84).

You can also try the classifieds on other websites, i.e. anibis:
robe longue de soirée

Thanks so much to all of you for your help! 

I finally decided to rent one, from Saint-Frusquin on Quai Gustave Ador: not exactly cheap, but they have a good choice, and some small sizes, which was what I needed...