You wont find any soundfonts that have that realistic sound point like slant'z. Your best bet is to get a piano VSTi, which is a virtual instrument. Has great sound quality. If your looking for a good program, I suggest either Sonik Synth 2 or Edirol Orchestra, which have great piano VSTi's in them. They arent exactly cheap, but are quick to find on bootlegg sites -coughs, u aint hear that from me tho. Just be sure to re up the soundbank on Sonik Synth.
Also, piano VSTi's dont just come with that type of sound, you have to get EQ's, compressers, and all kinds of other effects to tune your piano to have it in great sound quality. So look into that to.
Just droppin some help, peace.