My interest was sparked to do a little reading on this and from I can gather the sap is taken from very mature trees. I saw seeds for sale from several sites when I did a search on them although after reading about these trees I'd be leary to buy any as I'd be afraid they'd not be viable. This was a pretty good article about these trees.
Prized for Fragrant Resin, Frankincense Trees Are Vanishing
Deb