I'm about to get a 1968 delta 88 but it has a big rust hole in the front bumper. Where can I get a new bumper for it?

 

New bumper is highly unlikely. Getting an old bumper and having it rechromed is probably in your future.

 

There are virtually NO repro parts made for the Olds full size cars. These are not popular to restore and most were used as demo derby cars decades ago, so few remain on the road. Search wrecking yards in the desert southwest, then expect to pay $750-800 for a re-chrome.