According to the shop manual they're called "aero moldings", and they shouldn't be too hard to find, as there are a lot of these cars around. I've seen them on eBay, and you might try to find a salvage yard that specializes in Cadillacs. A and A Auto in Brentwood, CA (1-800-223-3971) for example might be worth a try.
Also, you might try posting on the Buy/Sell section of the Cadillac-LaSalle Club message board (
www.cadillaclasalleclub.org/). It's amazing what you can find there, and I don't think you have to join to post.
In any case, once you find them, you'll probably have to repaint them to get an exact match from panel to panel. Even when these cars were new it was not uncommon to find shading problems on adjoining panels.
Good luck!!