Stanley Garage Door Openers have not been made by Stanley since 1997. Stanley licensed the name to two companies after they quit production but those companies have both gone out of business.
Parts are hard to find but you can try Stanley Opener Parts
for parts and used units to repair your opener.
It’s probably not worth your time to track down the part, hope it’s the right one, wait for it, and then install it. For 150 dollars or so you can buy a new Genie, Craftsman, or Chamberlain opener. If you are not mechanically inclined you can have a Craftsman installed for around $109 usually the next day after you purchase it.
Check out a great selection here: Garage Door Openers This link is for Genie, Craftsman, and Chamberlain openers
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