Expounding on what Col Toda said, it depends on a lot of factors. There are a lot of possibilities limited only by your judgement and how you're running your campaign. Here are some ideas:
*If the the adventure is taking place on a backwater world, the Player Characters can see if there are any retired veterans they can levy from the populace
*On a more populated world, the Player Characters can put out For Hire ads, and interview applicants. Think Seven Samurai or The Magnificent Seven (really the same plot anyway)
*Would the PCs have enough time to venture to Terra or Outreach to see who they can recruit from one of the hiring halls?
*Maybe PCs just don't have the time or resources to properly recruit vehicle crews. Maybe they can haphazardly crew the vehicles themselves if they have the available manpower. Maybe they jury rig something so the vehicles are either somewhat automated, or are simply stationary weapon emplacements crewed by civilians. Or maybe they're rigged with shoe string to move forward in a straight line as a diversion.
IMHO, I think the last option would be more dramatic and more satisfying as part of the storytelling process rather than the PCs simply recruit vehicle crews.
Just some thoughts, hope that helps.