You are probably asking about sourcing original crowns for Longines Conquests? About all I can say is, if at first you don't succeed, try and try again!
Vintage Longines Conquests are not exactly rare. I wouldn't even call them uncommon. I wouldn't think a crown would be too difficult to source. Not easy - and maybe not cheap - but not super difficult.
Cousins UK has a lot of them, but you need to know the part number. It's difficult to cross-reference the crown from the case reference numbers for Longines. Boley.de has a cross-referencing feature on their website, but it's useless for the US market models. gatorcpa