You can get it online.

C28H30O4

Thy-mol-phtha-lin

Methyl Red, methyl orange, bromophenol blue, phenolphthalein, thymolphthalein, bromocresol green, bromothymol blue are all used as acid base indicators.

Disappearing ink is a water-based acid-base indicator (pH indicator) that changes from a colored to a colorless solution upon exposure to air. The most common pH indicators for the ink are thymolphthalein (blue) or phenolphthalein (red or pink). The indicators are mixed into a basic solution that becomes more acidic upon exposure to air, causing the color change. Note that in addition to disappearing ink, you could use different indicators to make color-change inks, too.