I have an old 30m rope you can have but it is grubby so not sure how appealing it would look as a mat. Maybe you wash them, I don't know? Also it may be useful for others to know what length you need for your average mat, I imagine you would prefer a continuous length for a mat.
Visit your local wall. They likely won't want to say in print (eg HERE!) that you can have old ropes cos everyone panics about getting sued, but a friendly chat or a £5 in their charity tin may work wonders. May also deprnd on how busy they are and which staff member you get...and if they know you as a regular.
Lorraine McCall began her attempt today at a continuous, self-powered round of the Grahams, a four-month tour of upland Scotland combining walking and bikepacking. If successful, she'll be the first person that we know of to do it.