Between Libreville and Mouila, most people have a couple of rusty oil drums sitting on the edge of the road (in front of their corrugated metal-roofed shack), showcasing wares for sale. The bounty may include coconuts, plantains, limes, or taro root.
If you see something you like, you're certainly welcome to stop and purchase it- maybe haggle over the price (though I can admit I'm not sure what these things are going for these days, and you'll have to figure out for yourself what to do with your prize).
Just note, it's probably better to pick it up earlier in the day rather than later.