Local autoparts stores outta have those rubber spacer things.
As far as the headliner goes, sometimes you can pull it down.. should be fabric and a backing board. Normally the glue that holds the fabric to the backing board has turned to dust, and it makes a real mess, so try and get it all out before you seperate the rest of the fabric from the backing board.
Once the backing board and fabric are removed from the truck, and the fabric removed from the backing board, clean off all the old dry glue.. which really shouldnt be any more difficult than brushing your had across it.. then get a can of that spray on glue.. spray the backing board, then put the fabric back on. It sets up quick, so have a paint roller with one of those big fluffy rollers on it and roll it all over the fabric to level it out smooth. Give it about an hour to finish setting up, then put it back in the truck.
Of course, if your fabric is jacked up, has stains or the typical cigarette burn holes, you'd be better off just replacing the fabric.. or if the whole DIY spray glue job seems like a little more than you wanna get involved with, several vendors sell replacement headliners.
