I’m afraid buying that special spare parts separately will make them even more expensive.
But of course I’m curious for a reply from Tektro…
To buy you would need a company which has it in stock - did you find any?
Did you see this?
http://www.rideon.fr/tektro/
Perhaps you ask also there and I try (via my bike-dealer) to get in touch with the German Tektro distributor Hartje.