Tristana, there are a lot of Nomex fabrics for sale right now on Ebay.co.uk.
You could also consider these alternatives:
1. Kevlar fabric - tolerates higher temperatures than Nomex and is much stronger. You can coat it yourself with high-temperature silicone to make it waterproof, if you want.
2. Fiberglass fabrics - tolerate higher temperatures than Nomex or Kevlar, but can fray and tear if it is abraded or flexed against a hard edge. E-glass is the common kind, but S-glass is occasionally available and it is significantly stronger and withstands even higher temperatures. These can also be coated with high-temperature silicone.
3. Basalt fabric - Very similar properties to S-glass, but brown in color and somewhat more difficult to find. I have used this to make parts for backpacking wood stoves. Also can be sealed with silicone.
4. Kapton (polyimide) film - tolerates temperatures above the limit for aramids (Nomex and Kevlar), but not as high as fiberglass. Inherently waterproof (it is a film), but not as strong as the others. I'd recommend Kapton film that is fairly thick (ie, 0.005") or laminated to a fiberglass fabric or scrim.
5. Pre-oxidized PAN fabrics - CarbonX is the most prominent brand right now. I have used this for backpacking stoves. It is 7.0 oz/yard and has properties very similar to acrylic fabrics, but withstands high temperatures. The only drawback is that long-term exposure to the temperature of a red-hot stovepipe will cause it to gradually degrade. You'll probably have to replace it every season or two. Here is a video demonstration:
6. PBI fabrics - PBI has the highest temperature stability of any plastic, and PBI fibers are often combined with Kevlar to make protective gear for firefighting. PBI/Kevlar firefighting hoods (made with "PBI Gold" brand fabric) are available on Ebay.co.uk, and I have purchased these for the fabric for stove projects.
All of the above can usually be found on Ebay. If you buy a Kevlar, fiberglass, or basalt fabric, be sure to get one that is pretty tightly woven. Most of these fabrics are meant for composite layups, and the weave is very loose with large openings between the yarns and poor dimensional stability. A heavy coating of silicone will stabilize a loosely woven fabric, though.
If you want to try S-glass or CarbonX PAN fabric, and you have trouble finding it in the UK, let me know. I can send you a bit (enough for your purpose, I think) of either of these for the cost of postage.