How would you find clothes that have not been manufactured by slaves ?
In Switzerland I can buy from Switcher (switcher.ch), which markets clothes "made with respect", and I just found out about the Fair Wear Foundation, which has a short list of member brands (www.fairwear.org).
As pointed out in this thread, just buying more expensive clothes may not make a difference.
She started it because she realised that in the UK marketplace it was very hard to buy underwear that didn't come from sweat-shops, even if you were prepared to spend a bit (or a lot) more. So these are made here in the UK, by disadvantaged women who get to learn a skill and take home some pay.