Did you check out
http://www.sickhids.com/ for custom lights?
Also, I've seen some other custom cars that used other cars light assemblies, I'm thinking that I might use Mini Cooper lights.
http://www.carid.com/2002-mini-cooper-headlights/
Unfortunately there aren't many modern cars that look like they will work with minimum modifications for a custom P1800. I've ben looking, the late model/retro Thunderbird lights might work too, but they would be way more work that the Mini lights. The Mini light assemblies just look like the might be a bit big.
I'm still working my tail lights for my 1969 Camaro. I have billet lenses, and I just can't spend >$200 for progressive tail lights. I found a company that sells tons of LED's to the trucking industry, and I purchased a set of progressive controllers and a few other pieces for the custom light's that I'm going to use from them. Should be able to make my own tail lights for just over $100.
http://www.rodworx.com/
http://www.uatparts.com/
These tend to focus on LEDs, running lights and tail lights, not HID's for head lights. I have not looked around too much for custom HID's yet, as I'm using the standard round headlights for the Camaro, so I don't have to look around too hard yet. I've looked around at the Good Guys shows and a few other shows and not much is available in those venues for custom applications.