I order stuff from them all the time in small quantities to a university, and a normal person's credit card should be much more convenient for them than dealing

I order stuff from them all the time in small quantities to a university, and a normal person's credit card should be much more convenient for them than dealing with procurement beurocracy, net90, etc. The plastic scintillator is chemically just plastic so there are no safety complications shipping or receiving it. You might have more trouble getting large amounts of liquid scintillator because of the solvents. Another thing you might consider is scintillating fiber with wavelength shifter. If you read out from both ends you can estimate position within the fiber by timing and possibly amplitude weighting.