The most fairtrade and natural way to get hold of wool is to find someone who has some organic sheep (btw even 'organic' sheep are usually dipped in something once a year but a responsible sheep-keeper will know what chemicals have been used). Sadly, there is no money in wool in the UK and most commercial flock fleeces are burnt because the cost of collection by the wood board is higher than the value price a farmer can get for a fleece. Therefore, organic fleeces are more likely to be found on the local crafters scene than from any wholesale fabric supplier. Unless you import from Eastern Europe or a 3rd world country and then the organic and ethical standards are hard to verify. (I have worked in a fabric shop over many years and the fabric trade is just as bottom-line and riddled with environmental and ethical awfulness as most businesses.)
So, to find your local organic pet-sheep farmers, I'd suggest contacting your local guild of spinners and weavers. I know my local group keep a register of local people and the breeds they keep. Once you have your fleeces, you will need to find a willing weaver. Be prepared to pay a good hourly rate if you want to treat your workers with a fair-trade ethic. The minimum wage is not enough for skilled craft like this in the UK, so be prepared to pay at least £10 and more like £20 for the time spent spinning and weaving your fabric.
I once did the maths for a handspun fairisle cardigan. At the time the most you could get for such a garment was about £75. It can take a competent spinner just over 70 hours to prepare, spin and knit a complicated design like this by hand. So the hourly rate to make one worked out at less than £1. There is a reason why there is no market for this kind of fabric. I imagine at a fair wage, your robe will cost many hundreds of pounds, possibly more. Which begs another question - is it ethical to spend that much on a garment....