Years ago I saw someone had cut up pieces of doormat and used it as cornfields for their American Civil War games. Now the problem with doormats is that you can rarely find one without some sort of motto in black or red. None of these any use at all whether they simply say "Welcome" or "Husband For Sale".
You want coir matting, although I've seen astro-turf used to good effect as well.
All you need is a strong craft knife to cut through the rubber underside of the mat. I don't recommend using scissors!
From a standard door mat you should be able to get half a dozen decent sized fields. Recently B&Q in the UK have been selling these for £5.
Here are some door mats deployed as cornfields: