The cheapest place for the green sub-battery (FRU 93P5001, and you
will need the part number) that powers the ThinkScribe is IBM themselves. I bought two from the parts place in Boulder in the past year (nothing wrong- I own two t-notes) - they were $16 plus shipping. I've seen places asking as much as $90 for them.
The IBM site also showed the yellow CMOS batteries(FRU 02K6682) as being in stock, but they always turn out to be on back-order. I ordered those from a company called
www.indexworldwide.com for about $20 each. HTH.