I agree - you would have to scour it yourself. Really you are only going to get scoured fleece from one of the mills, say around Bradford, and it won't be cheap. Remember it will need to be somewhat compacted for use as insulation, then treated against vermin, or you will have a roof full of rats.
I think you need to do a bit more research, for example how many fleeces you will need to cover that area to that depth. At a rough estimate, a rolled up fleece, which would give you the depth and degree of light compaction you need, would cover about 30x45cms, so 6 fleeces per sq metre, 1,200 fleeces - that's a big farm and that number of fleeces will already be contracted to be sold to BWMB. Different sheep breeds have different weight fleeces, and some fleeces would be more use as insulation than others. I think the only situation where it's cheaper to do it yourself is if you have your own flock of sheep