I've been thinking about this same thing recently (black vinyl for the outside of the pillars).
Just today at work, I came across squares of matt textured vinyl stuck on the walls in the museum gallery that I was dismantling - covering up various holes! - so obviously we have had a source of this in the past (it was also used to mask around CRT screens, which are now obsolete: we use LCDs). No-one seems to know where the matt textured vinyl came from, but my investigations are continuing.
It is the same stuff used on newer cars like the Punto and Stilo. It is not totally practical because any wax residue (from polishing) is hard to remove, and like all vinyl it tends to shrink/crack/peel from the edges. But it should last at least five years.
In the meantime, you could use the easily-available smooth black vinyl which is probably more practical (it can even be polished!) This is what custard boy suggested - it is used for signwriting on vans etc. Sign-making shops sell this by the roll. It is probably 10-15 pounds in your money for a lifetime supply... that is a fraction of the price of anything that Halford's would sell
We have a vinyl cutter at work, which is a magic machine made by Roland. One of those excellent examples of computing technology at its finest - now about 15 years old and running off a Windows 3.1 system with Corel Draw. The cutting tip is a tiny stylus that looks like a sewing machine needle. The stepper motors are seriously powerful and make a wonderful sound. The hardware seems to be designed to last forever, none of this limited-lifetime high-consumable rubbish we have with printers these days
It must be worth its weight in gold over all those years, used for lettering and graphics on all the museum gallery walls. It is shortly to be replaced by a newer machine, but I expect it will be around for me to use for some time to come... It works with full-width rolls of vinyl (1m wide, I think?) and can produce a square metre of 72-point text (one inch high) in about 20 minutes. I'm considering using it to make the 'Turbo i.e.' lettering and sidestripes for the Mk1 Uno Turbo, but of course need to get the design into Corel Draw first.
-Alex