One of the best places I LOVE, LOVE, LOVE to shop for my daughter for Gap jeans is Ebay. They have all different designer brands for any size or gender, and I always get a great price over what I would pay in the store!
As I don't know where you are located, www.bluefly.com and www.emoda.com both sell designer clothes online, and will deliver to your house. A local mall is a good place to find designer clothes. There are also multiple designer clothes retail stores in every city.
Nordstroms, Saks, Bloomingdales, Dillard's, pretty much anywhere that sells designer. They only wear designer clothes!
Balardi sells luxury designer clothes online, their uniqueness is that the prices are extremely competitive. There is a big selection of over 7000 items on sale, 95% off them will be cheaper from Amazon, eBay, etc check out this link.
Ebay or Amazon usually sells them for cheap.
A thrift store sells some pretty nifty clothes for low prices. It will look great, and nobody will know where you got it! I'm sure everyone will think you look great.
Why don't you try vinted if it's available in your country. Here's a link to the site vinted.co.uk/invite/ade316
places like designer depot, or an outlet store
in the UK Republic sells bench clothing you can order online also
You can buy them from either Cult or Superdry Online.
You can find it easily. Recently I bought Some apparel from eAnythingindian that was nice and the fabric of the clothes was good. I like shopping here. You can also try and I sure you will not regret.
You can find a store that sells fluorescent clothes at specialty clothing stores, online retailers, or stores that cater to rave or festival fashion.
Amazon.com is one place which sells a wide variety. After that the best idea is to search for the best price using Google Shopping or Nextag.com.