I ordered some horsehair canvas, body canvas and collar canvas from a place called Textielstad. The shop is miles away, so it was an online purchase. It's worth me checking it out because buying from Kenton or Bernstein & Banleys takes longer and the postage cost is higher. I also bought from a German supplier, but it turned out to be different from the advertised product and they were unhelpful via email.
Anyway the cloths arrive on Tuesday or Wednesday, so I can asses them. It may be a good source for people in NL, Belgium etc. The cost is quite low too with the haircloth at just €6.50 rather the £9-10 I would pay at Kenton plus a sizeable postage fee.