You can buy simple mats, like the one Barbelboi linked, pretty inexpensively. However, it's worth thinking about how you are going to carry them. I know many folks fold them into a chair. I often don't take a chair so they become a pain to carry. I also tend to prefer a mat with some sides, particularly on sloping river banks.
Without getting into the realms of silly money, and excessive sizes, commonly found in the branded carp tackle, there are good and more versatile options to look at. The downside is that they get more expensive.
I have had one of the old style Korum Multi Mats for many years now. I use it to carry a few odds and sods (banksticks, usually), but mainly as padding between me and a shoulder carried tackle bag or chair. Unfortunately, it would appear that the price of the
latest incarnation is at least a tenner more than I recall the original being. However, it does seem to have a few extra twiddly bits and pockets. If they suit the way you fish, they could be worth paying the extra for. There are also a few more combination and plain mats in the Korum range that might be worth looking at.