Do you use them? How do you use them? How can I get some?
I have not used them. The amount of hops I need for my beers is never high enough to justify going for the extract. But if I wanted to make a super-high IBU high-gravity monster, I'd think about it.
Northern Brewer sells them. Seems like you can add it at any time during the boil. And the NB site links to a calculator you can use to determine IBUs.