Most injection moulders will sell you or give you a bag.

There are at least 5 such companies within walking distance of my house and I don't think I live anywhere unusual.

The cost to them is peanuts and generally if you want to experiment with plastic they are interested enough to give you a shovel full. Just pick the small one man bands or companies with three or four employees, where the owner actually works and you can learn an awful lot about different sorts of ABS or whatever and walk away with enough to keep your average 3d printer going for 2 months.