Hardware store or a garage door sales/repair organization. You need a
good stapler/tacker and LONG staples. The stuff comes in a couple of
different widths, so get the right one -- either measure, or bring a
sample. The width is the bottom of the door, NOT the seal itself. Make
sure the bottom of the door is clean, etc., so that the seal fits
well. Total time, maybe 1/2 hour, not counting the trip to the store.