Search for a fencing supplier or farm supplies, may have a local one, otherwise Wynnstay and Mole Valley are some of the nationals.
If you don’t need a full roll, bung a fencing contractor some cash for a part roll. Or might even sell a whole roll, they’ll be getting a good price on it as they buy bulk.
Edit to add that farm supplies aren’t great at online, lot of stuff is assumed that they have, so give them a call or drop in. You won’t see it in the showroom, it will be in the yard out back. Heavy stuff, especially the 200m rolls. 50m an easy lift.