Open Discussion Group

Yes, they are the only readily available tyres. Bridgestone still make a 17 inch tyre for the Bridgstone Moulton, which itself has ceased production, but I think the tyres are still available in Japan.

Bridgestone still stock and sell the 17 x 1 1/4 (32-369) tyre, as I ordered and bought 4 on a recent trip to Tokyo (4500 Yen each). A subset of their bicycle shops labeled Active Shop on their website seems to be the secret to sourcing them, though you may need some help with their website to get their address as the relevant list of shops is in Japanese (ignore the English button, it takes you elsewhere).
Starting at http://www.bridgestone.co.jp click on the image with a bicycle; this opens http://www.bscycle.co.jp and when a red banner with Bridgestone Cycle Active Shop scrolls past, click on it; top right corner of the next page is a map of Japan with magnifying glass – click on it. There are 5 columns of prefectures, and at the bottom of the furthest-right column is Tokyo – hover on it and see Tokyo highlighted on the map, click and then a list of areas appears – 4th row down and 2nd along is central Tokyo. Click that and a map appears marking the shops with corresponding details in Japanese on the right. You can move the map and it updates with shops in other areas.
You might need to allow a week for the order (for which you pay up-front) to arrive but the shop I used (near Morishita on the Toei-Shinjuku Line) received it in half that time. The shop owner did not speak English but recognised the word ‘Moulton’, though I cheated as my wife is Japanese.