I've been trying to find website with some Sith robes that look as authentic as possible, know any sites that have some?
Try Etsy and Ebay for sellers who make custom robes. If you're in the UK, try www.jedi-robe.com; they have a SW store based in London and an online shop. A friend got his jedi robes from there and they look fantastic.
Etsy and Ebay are both reasonable bets for finding people. I would also recommend looking to either the Rebel Legion or 501st to see if you can find someone who takes commissions- I know there are a few folks in the Rebel Legion who do custom Jedi, and depending on the look you're going for, a custom Sith is just a matter of a color change. Or, there is always the option of buying a sewing machine and learning to do it yourself- basic robes are a great project for beginners, and that way you'd be able to get the look you wanted without having to depend on someone else, so keep that open as an option.
The Sith robe I have now was made by relatives of someone I knew through school, but they're not at all professional and I'm planning to get a higher quality Sith robe, as well as a Sith tunic, in the near future. For a robe, by far the best of what I have come across is this: http://www.mystikmerchant.com/MASTERSROBE.htm Someone who has been doing this professionally for many years and they're high quality and machine washable. For a tunic, I think I'm going to go with a karate gi. It definitely seems to be the most accurate, and will probably be higher quality than something intended as a 'costume'. The only issue is that most have some sort of logo or something sewed onto them, so my issue is finding a supplier that doesn't do that. These ones have logos on them but just to see how similar they look to a Sith (or Jedi) tunic: http://www.toukaido.jp/ProductsDetail.aspx?ccd=TKD01&pcd=SAB http://www.shureidousa.com/gi/k10tk10kb10tkb10.html http://www.shureidousa.com/gi/kw11kb11.html Personally, I'm going to leave it at the karate shirt/pants and the robe. If you want to have more layers and have more of a Jedi robes look, you'd need an obi and tabbard, and likely one if not two more full layers. If anyone knows where I can get a high quality black karate gi without any tags, logos, etc, please PM me. Thanks.