Just a guess Angelo; but I doubt there is any money to be made in selling pen parts.....especially parts for inexpensive kits like the slimline. I suspect that Woodturningz does this purely as a service to their customers and it wouldn't surprise me if they didn't make a penny off of the transmissions....might even be a losing proposition??
Just to put in my two cents, both PSI reseller here in Montreal sells them so i suspect that any PSI reseller (like Woodturningz) migh be able to get them. Check with Pens of color, wich is also a PSI reseller.