I'm going to start calling round bike shops in the am, but if anyone has suggestions for sources, I'd appreciate it...
Call Parkers first?
Simon, thanks for the suggestion. I did call Parkers, but although they can order the replacement band for Tacx rollers, they didn't know what size the tacx band is. I also posted this problem to uk.d-i-y (who I've found to be the most useful bunch of all-knowing helpers in the past), and one of them put me in touch with an O Ring manufacturing company who have a range of bands or about the right size for £2 -£3 each. I'm going to try them first.