Given that the included providers do not update their prices frequently, it would be appreciated if additional providers that update more quickly could be provided as options.
TCGPlayer offers an API that feeds their low, middle and high prices: http://store.tcgplayer.com/faq.aspx#Q9
An example implementation can be seen in the top left here, the "L: M: H:" section: http://magiccards.info/query?q=tragic+slip&v=card&s=cname
I imagine these would each have to be implemented as their own provider, but I am not sure.
In addition, StarCityGames is one of the most frequently used price providers and offers up-to-date pricing, albeit not as nicely. For example: http://sales.starcitygames.com//search.php?substring=Sulfur+Falls&auto=Y