How to find a piece of matching carpet?

111222

Have put a mark on carpet, i think i can just cut it out and fill in with a section of the same carpet....

How would i go about find a small section to do this.....it's the kind of carpet that you could be able to do this with just the material you wouldn't notice the it had been done...

Errata

If you can't get it from the shop the original carpet was bought from, and if it's more than a year or two old it may no longer be available, you could try cutting the piece out from underneath an item of furniture that will always stay in the same place, like a sofa or bookcase.