Long Flowing Dresses
If you are looking for long flowing dresses (for fantasy cosplay or other reasons) for Spring or Summer then one of the websites I strongly recommend is ETSY. Handmade is always best.
Or maybe you just like retro clothing. In which case I strongly recommend you check out: Classic Gowns and Dresses
Or maybe you are just bored of little black dresses.
Whatever your reason there are lots of places out there to go shopping for find long flowing dresses. Zalando.co.uk for example has quite a collection of 250 dresses to choose from if you live in the UK and want to find a dress that is locally made. There are similar websites out there, but very few will have such a quality and broad selection.
Or you could make your own dress... not necessarily using old t-shirts, but a trip to your local fabric store might give you ideas.
Furthermore a long flowing dress is versatile for both formal and informal occasions. Works just as well for a friend's wedding as for a picnic with your favourite boytoy.
(And for the more nerdy women out there, long flowing dresses are also very useful for cosplay and LARPing, but that is another topic for another day...)
I recommend people focus on purchasing dresses which are easy to clean. Or if you do buy something that stains easily (ie. silk or satin) then here is a hot tip: SCOTCHGARD for Fabric. Its true they make the stuff for furniture, but they also make the stuff for clothing fabric too. Meh, accidents happen, so it makes sense to protect your really expensive dresses.