3.1 mpi. Got a vacuum leak that pops up every now and then and I have to push this rubber piece back in. Where can I find one

 

You could try the Chev dealer but I doubt they will be able to find one for you. I had to replace mine due to damage to one of the tubes so had to go to a wrecking yard and pull one off a donor car. You could try a little grease on it to get it to slide in further and then use a tie wrap to hold it in place. I believe one of those "plugs" is a dummy plug (if I remember correctly) so you could put a little black RTV on that plug to hold it in place but that might be a problem if you have to take it apart again.