DIY blackwater extract is actually very easy to make... i have been making my own since i was 12. find hardwood trees that form thick bark, like oak trees. the denser the bark, the better. find some of the bark that is easy to collect(the bark on rotting logs works great) and collect enough to fill about half a large coffee can. take them home, soak the bark in a pot of water on the stove, on low, for about half a day, then grind them up in a coffee grinder.
if you can find half rotten logs, you may be able to skip the soaking step and go straight for grinding it.
pour the ground up bark in a pot and boil it in water for a couple hours. strain the bark out, and you have blackwater extract. i like to go a step farther and take the gallon or so of blackwater and concentrate it by letting it simmer down. i like it to be as thick as motor oil.
ground up oak bark in a pan:
what it looks like in a glass, after it concentrated to the point of beginning to crystallize:
what a tablespoon does to a 40B. this pick was taken with a smartphone shoved right up against the glass: