any tips on where I can source some rear passenger footwell mats that would fit a 12 superb as cheaply as possible? not going to spend more than I need to something not to my benefit.

If you want the original basic version of the OEM mats, you need to get them from a breakers car. The original mats are only sold as a full set.

 

The other trick is to get the set of rubber "cut to fit" car mats that places like Halfords sell for £10 to £15.