Occasionally scrap pieces will wind up at the ReSource Sales Yard out near Stazio- they have a ton of used home construction material. I got some chunks a couple years ago for practically nothing to finish off my skylight. May be worth a try if you are in the area.Drive around and find a job site, someone will give you a scrap piece!
If not offer them a beer!
My friends told me to go to McGuckins in Boulder and ask for 'bitch tape'. But I figured some Amazon would just beat me senseless. So I didn't. This info will really help.Construction type places. Home Depot, Lowes. It's a roofing material. Ask for Grace (thats a brand) ice and water shield (product name). There are probably similar generic products that cost less than the Grace products.