Hi David,
The "Symbol" font mentioned by Suzanne is one possibility.
Depending on the font you are using, you'll also find some or many
mathematical operators in the subset "Mathematical Operators", starting at
U+2200.
"Therefore" is U+2234 (type 2234, then use Alt+X), but it's only included
in some larger Unicode fonts like "Arial Unicode MS" or "Lucida Sans
Unicode".
Regards,
Klaus