He didn't lie to you. The entire assembly or just the housing and lens are no longer available through GM. The entire assembly would have been about 90 bucks new thru gmpartsdirect.com, and the reflector housing and lens by iteslf would have been about 50. Just a guess by the numbers I have here.
There should be a ton of these things at a wrecker. Go to a wrecker that has a locator service. They're out there. You shouldn't pay more than 30 bucks for a used one. Print this post and use it as a bargaining chip!
Your part numbers to help with your search:
Headlamp assembly (all parts including housing/lens, aiming screws, plate), right side for 1987 gm pn 16505632.
For just the housing and lens (which you probalby need....not all that other stuff) right side, it'* 16516770.
1987 was the only year these lights were used on the Bonneville, BUT other H bodies that year may have used them.