It depends how much you need, and how often. I also ship stuff (I'm an online bookseller) but in small quantities. I get all my bubblewrap free from a variety of sources.
(1) my local Freecycle;
(2) local supermarkets line a lot of veg/fruit boxes with it;
(3) furniture often arrives protected by it, so I keep an eye out for office refurbishments which often chuck out stacks of the stuff.
(4) I always inspect any skips I pass - you never know!
Hope that gives you some ideas.