My Bonneville was refitted with air hydraulics. The system was near $3000 and all custom made. If you really wanted to lower an '88, I'd cut the springs. Just about anything else will cost you more than the car is worth.
Bonnevilles are a full sized 4 door sedan. Unfortunately Aftermarket companies aren't interested in making parts for a very select small number of people who want to customize them. If you chose to be one, you have to expect to do a lot of fabrication and spend money.
All the way up
Down as far as possible without scraping the front air dam and splash gaurds.