They probably have their favourite sources, but there are TONS of stands at the NEC during Crufts. Perhaps they stock up there! I'm sure they will be found at most of the general Champ shows up and down the country.
They are not rubber bands, they are silicone -used a lot for Poodles, plus as the base for many bows. I have never looked for the Yorkshire Terrier bows but most dog grooming supplies websites and also Amazon have the bands.
The bands for the topknot are latex. You need plain bands to build the topknot aswell as the bow. You can get them and maintainence wraps from here... https://www.hubintsecured.co.uk/acatalog/Bows.html