How can I find mutations associated with disease in human histone residues?
Ensembl contains this information: When you go to the “phenotype” menu item of a given gene, you will see a list of variants (potentially after clicking on “ALL associated variants”) with their associated genomic position.

You can subtract the transcript’s start position from that position to find out the residue (of the translated sequence, and be careful to subtract the lengths of all preceding introns!). Unfortunately this seems to require some manual work.

Alternatively you can query the individual SNPs, which will tell you their position in all transcripts (in this case: Leu 298/68/244):