That's what islanders do wherever this tree grows. I first encountered this trick in the Marshall Islands. A walk on the shore there will reveal thousands of brushes with soft silky bristles. Locals told me to make sure the brushes are clean, since they are also used for toilet paper there.
Step 1: Find a Pandanus Tree
Some varieties of pandanus are bred for especially long strong leaves, just for making matting. Some are sterile and have no fruit. Those are grown from cuttings.
In Micronesia they've bred some varieties with huge fruit. I've seen some bigger than the biggest watermelon. I've heard they can weigh more than 100 lbs.
I picked up a bunch of old keys under this tree by the Mo'olele canoe shed in Lahaina, Maui.
Step 2: The Fresh Fruit
Another traditional method is to roast them, pound them and wring out the juice to dry into a sort of fuit leather.
Step 3: Card the Bristles
Rub the bristles on a wire brush to soften them up and dislodge any loose ones.
The best paintbrushes come from keys that get beaten by the surf and then wash up. Watch out for sand that comes out of the inner part of the key. A trip through a washing machine would probably be good for the brushes regardless of origin.
Step 4: Nature's Disposable Paintbrush
I feel bad about throwing away a commercial brush every time I glue something.