Nope!
The valence changed on 1988 cars, but they were certainly aluminum bumpers in the United States.
The valence change was done because euro market cars DID get the plastic bumpers in 88.
1988's were a transition year for the front in the US... the valence integrated the foglights, the tail lights got taller, the headlights changed from sealed beams to "ellipsoid" projector style.
The valence change meand that it's possible to swap the 89-91 plastic bumpers onto the car, but 1988 cars in the US never came stock with plastics.
OP, there are loads of e30's out there. Keep cash in hand and be patient if you want to bag a good one. As you know with your e28, very few 25 year old cars are "road worthy" so be ready to improve your car, or be ready to pay up for an enthusiast car.
EDIT:
op, just one week ago, here is what I found on CL, locally:
http://eurowerks.org/showthread.php?...402#post724402
some of these are still for sale. The others were scooped up