As diaglo mentioned the artist, Keith Parkinson, died some time ago. There are a few items still available on his website (run by his family, I believe) but Lord Soth's Charge is not one of the posters they still have in stock. They do have it in giclee form but it's quite expensive as giclees tend to be.
As well as Art of the Dragonlance Saga there was also a 2nd art book called Masters of Dragonlance Art, which is also available for cheap on the used market. Not as good as the first but still worth it for the right price. Covers later material than Art.