When employees leave, they create holes that might be filled by temporary employees -- especially ones with a few decades of experience who want flexibility and challenges and who cost less than a full-time hire with the same qualifications.
First, figure out where the holes in your company are. Then, take a look at your company's recent retirees. They might be interested in coming back for a project or a temporary position.
Also consider placement agencies such as Rent a Geezer. This part-time solution could help you buy time until you find full-time replacements.