Unicode arrows heads:

  • ▲ - U+25B2 BLACK UP-POINTING TRIANGLE
  • ▼ - U+25BC BLACK DOWN-POINTING TRIANGLE
  • ▴ - U+25B4 SMALL BLACK UP-POINTING TRIANGLE
  • ▾ - U+25BE SMALL BLACK DOWN-POINTING TRIANGLE

For ▲ and ▼ use ▲ and ▼ respectively if you cannot include Unicode characters directly (use UTF-8!).

Note that the font support for the smaller versions is not as good. Better to use the large versions in smaller font.

More Unicode arrows are at:

Lastly, these arrows are not ASCII, including ↑ and ↓: they are Unicode.