When I was seeking similar I had found a company called "BOCA Bearings" in the United States. I never got round to buying any though. Too bad we're in different areas as I would have gladly put in a joint order.
Fair Winds!
I have just obtained some (500) which are 3/16" diameter - for Sailspar "medium". If you send me your details I can post you some. I got mine by sending an email to an ebayer who was selling them by the thousand.
Ball bearings are also available in various plastics, Delrin is OK for moderate loads or for extreme performance, Torlon. Google will give you plenty of options.
Thanks to all who replied. I got some at the rigging company at the marina. Fifty two at 58 pence EACH! Plus VAT!!!
Got them in after about twelve attempts and the roller is better but not perfect. If I do it again I will remove the housing (but how?) and renew the thin stainless steel shells (but where do you get these?)