Methyl red, called C.I. Acid Red 2 (2-(N,N-Dimethyl-4-aminophenyl)azobenzenecarboxylic acid), very red at pH 4.4 (and below), remarkably orange at pH 5.1 and bright yellow at pH 6.2.
Look in microbiology sales places for C.I. Acid Red 2, it is principally used to identify fecal contamination
. Handy stuff to have about the house.
In IMViC test Methyl red is the “M” part of the test.
I am sure places like Carolina Biological carry it (I really like Carolina Biological,
an unpaid, unsolicited endorsement). Sorry for the delay, just checked and my Methyl red is definitely Carolina Biological.
As it happens I use Bromocresol Green - Methyl Red, Aqueous, for Alkalinity when measuring carbonate hardness.