Find a system in Famine, go to one of the Distribution Centers (you will see them in your Nav Panel).
Scan the wakes of the ships jumping out, might take a bit to get however many you want and/or need. You will get other data while you are there, some of which you might also want.
I recommend a fast, nimble ship, but any will do the job.
Edit: My bad, reading failure. Forget the above, unless you are looking for Datamined Wake Exceptions.
DavMart is where I get Chemical Manipulators, unless Frontier stealth-nerfed it in the last week or so.
Riôt
Actually this is of some use OP. Do what he said (fly to distribution centres in systems where the controlling minor faction is in a famine state) only instead of scanning the wakes of the ships that jump in, blow up the T-9s that are sat there providing supplies. Obviously, do this in an anarchy system...
Dav's Hope is hit and miss. Best I've had there is two lots of chemical manipulators on my first drive round the circuit, worst I've had is over 25 relogs without getting any.
I believe the mega ships like the tanker mentioned above are the best way of all now but unfortunately there's no way to tell where they are using in-game resources (because that would be both useful and easy) and the Canonn website wasn't working last time I looked at it.