[Help] Trying to fix the button/popper on my coat Sewing Machines Parts and Sales in Los Angeles

Did the snap separate or did it rip/tear out? If it just separated, then the snap just needs to be replaced (including its mate). If it tore out, you should reinforce the fabric (with another piece of fabric, go figure) and then replace the snap (again with its mate).

In addition to dry cleaner or local seamstress, you could check with a cobbler (shoe repair place). They often tackle snaps and blue jean-type buttons (which are more or less a specialized snap anyway).