I use phone book paper for my string blocks- there are always a stack of them around at work that no one uses (need a phone number? just Google it!). If I need more precision I use printable foundation papers- pricier but I buy them when I have 40% off coupons or when quilting notions are half off at Joann. They even make some that dissolve, but I keep larger pieces of the tear-away to use again for smaller projects.