After looking at WIKI
trisodium phosphate
did not look so good and the MSDS scared me
but diluted it might be OK
But
From WIKI they had this list
Sodium triphosphate, a water softener used in dishwashing detergent and a food preservative.
Sodium dihydrogen phosphate, commonly termed monosodium phosphate, (NaH2PO4), is a laxative a pH buffer
Disodium hydrogen phosphate, commonly termed disodium phosphate, (Na2HPO4), is a laxative and anti-caking agent used in food.
Sodium aluminium phosphate, (Na8Al2(OH)2(PO4)4) is a pH buffer and an emulsifying agent used in food.[10]
Thus the following seems to be good candidates
Sodium dihydrogen phosphate
Disodium hydrogen phosphate
Sodium aluminium phosphate
Also WIKI says
citric acid can possibly be used as a buffer