See terrypchan's post above. At least in my games, it's been sold by the lady in Debir who gives you your pet dragon.

It can be sold "anywhere." However, the most common area is, as you said Martha, the lady in Debir.

Also, it is possible she will only sell it after you have returned more stones (2nd, 4th, 6th, etc).

The scanner actually picks this up too.

In my game, it was (and still is, I am not using that +1 attack thing...) on Bolo, in the tent behind the Barbarian trasher.

Well in my games it has always been sold by Marta

Ugh I can't believe I missed that. I didnt talk to Marta figured she didn't sell anything. Too bad I'm already at Verona so not 'that early' in game anymore.