Font that has characters for Nez Perce Language (x with a dot under it) Nimiipuu or Nimipuutímt

The alphabets and overall number of glyphs in a font file will vary widely from one type family to the next. Some system fonts, such as Arial in Microsoft Windows, may have thousands of glyphs. Fonts in other type families may have a bare minimum of glyphs to only cover the Latin alphabet.

Even Arial doesn't appear to have a "X" glyph with a dot below it. The glyph may not have a Unicode designation. Native American alphabets aren't well supported in digital typefaces. It looks like some people are able to create it combining a dot below mark (U0323) with characters above it.