I got the spots from a warehouse style shop in Brick Lane, as suggested by DIYCouture's wonderful fabric shops guide.
Here you could buy cheap fabrics for £3 or so a metre (minimum spend £12 - but that wasn't a problem!)
Lots to choose from in Epra Fabrics. Next came cutting out the squares, something else I did a couple of weeks ago.
Finally, the normal sewing and sandwiching to make the finished product.
The finished product was a little larger than planned - not quite the snug envelope of the others I had made. I blame slightly mushed 'ill-brain'! Still, it means I can also carry the cable with the laptop inside the case - so it could be an advantage after all.