I have an old cheap guitar that I just use for the beach etc., that plays perfectly well, but the roller on the sixth string has a groove cutting almost all the way through the string hole from years of string pressure. But the other rollers and the tuners are fine. Is there a way of just replacing the one roller without buying new tuners? And where can I buy a roller?

This in the past was very easy to find as it happened sometimes. But of course you need the exact size.
The other possibility is to take of that machine head and glue the roller with very hard glue here in UK called Araldite. I think it will be as hard as the roller when it is dry in maybe 48 hours.

Hi PC you can buy seperate rollers from this site here in the UK
http://www.touchstonetonewoods.co.uk/ttwp30.html