Radio Spares changed their name decades ago (1970's?) to RS Components (as they moved from spares for radio/TV to Industrial), and very recently to just RS - and I don't think they ever did drive belts anyway?. Most of the TV/Audio spares suppliers have long since ceased trading, and a google search such as the address from rjenkinsgb is probably the way to go.
Even back when the spares suppliers were still going, it got to the stage that almost none of them kept specific drive belts, what you could get was a pack of belts of different sizes for VERY little money. Cost wise it was great, but it was annoying, as you almost always only ever used one or two sizes - I had a big draw full of the sizes that were left over, as I couldn't bear to throw them away.