You can get the 16" tubes from thorusa. He sells both presta and schrader valve style.
when I had to replace the 12" tube on my Frog, I couldn't find any local sellers for wheelchair tubes and ended up buying a 90 degree valve adaptor. It has come in handy for also pumping my 18" birdy wheel that has a hub motor http://cgi.ebay.ca/ws/eBayISAPI.dll?...B:EOIBSA:CA:11
Yes, the Mobiky size is 12½ x 2¼ (ISO 203mm). I think of them as 12" because that is their actual rolling diameter.
Thanks for the link to 1800wheelchair.com. They even have a choice of a 45° or 90° bend in the stem on the 12.5" tubes. I am guessing that in this case 12.5 is the same as 12½ though that is not a good general rule. But no 16" tubes.
I do not see any tubes with bent valve stems on the Thorusa site.