Ant, all depends what you want. I used to use the living room carpet, when I changed to laminate floor bought one from Argos for £12, same one offered by golf retailers £30 to £40. 9foot long with a slight incline to the hole, helped me with alignment and not leaving the putt short.
http://www.linksputt.com
Found above link prices start at just over £200, looks Ok endorsed by Harold Swash & Monty.