I have just noticed the response about the 45T TA ring on Ebay. However should Dan require something even smaller, I remembered that SJS Cycles, a touring cycle specialist based near Bristol in the UK ,has tried to build up a stock of these rings.
The current stock on his website shows outer rings available in 30, 31, 32, 36 and 42 T sizes. Priced at £24.99
The shop's site is on:- http://www.sjscycles.com/
Hope this proves useful
Norris Lockley..Settle Uk