After reading this question, Double stroke fonts with unicode-math like dsfont, I remembered that the font which comes with OpenOffice (Linux Libertine G) has a beautiful (but I don't know if useful) double stroke alphabet.
How could we use Linux Libertine OpenType with XeTeX and take the double stroke alphabet from that font.
You can see how is the double stroke alphabet in this document, at the bottom of the second page.
{libertine}
and[libertine]{newtxmath}
).