Unicode symbol that represent "download"

Check this site out. You might find what you need. http://www.alanwood.net/unicode/arrows.html

For example this: ⇓

Although an image would be a better solution in my oppinion.

You could use ↓ (U+2193), it is available in Arial font. To see a complete list run charmap.exe from windows. It will show you all unicode characters available.