What I often do is frequent local Flea Markets, and Thrift Shops. The other day I was able to pick up a container with a estimated 3000 parts for only 20 dollars. (Keep in mind half of the parts are yellowing dirty or...covered in a mysterious substance) I find this the easiest way to buy in bulk. However while sorting trough bins you can sometimes find unwanted "Stuff" other building toys, used Band-Aids, even a plastic bag full of leaves. Its not the cleanest way or the easiest if your moc building ,but its easiest way in my opinion