Look for round polyurethane belting. Widely available, eg try ebay.
IIRC, mine is 5mm. dia.
It's worth taking a little trouble over joining. Heat a suitable piece of mild sleel, hold both ends in flat contact with it for a few seconds, then push together firmly. Takes a bit of a knack to get the joint well aligned, but practice make perfect!
Should add that cutting the ends as square as possible with a sharp blade aids a good join.
Edited By Clive Brown 1 on 08/07/2018 21:44:59