To my knowledge, there is not an official "Eilonwy" doll. There are people who have made custom ones from other dolls and these are the ones you find on Google. I know that there are a few official Disney pins of her though and a "Magic Memories Figurines" group from the film that has her in it.
There is very little Black Cauldron merchandise in general. While the movie did poorly, I think that the lack of merchandise originally has more to do with when the movie came out. The "Black Cauldron" was released in 1985, so early on in some ways of all the great toy marketing pushes that came later on. We get so used to them in today's world, that we forget that it didn't used to be like that every time a film was made. Now the lack of merchandise in later years (other than a few pins) is more than likely because of the lack of interest and perception. It did eventually get a VHS release FINALLY in 1998, and then the 25th anniversary edition on DVD in 2010.