In order:
1. No, the newer model grips will not fit.
This is due to the newer guns having a short "stubby" grip.
Here's a picture of the difference:
Newer style grips overlap on the bottom to make the wood longer.
Here's what they look like:
Note the overlap on the bottom:
2. Older style Police Positive Special grips, and any of the older pre-1960's grips will fit, since these older guns had the longer grip frame.
3. After WWII, Colt began to assign letter codes to their guns.
The Post-WWII small frame revolvers were all "D" frames, including both the long and short grip frames.
4. I have no idea. Most makers that made the older long grips are either out of business, or discontinued them.
You can watch Ebay or start contacting grip makers. Some will no doubt make a custom set for you.
Eagle will, I think, OR they may still make them for the older, long grip frame:
http://www.eaglegrips.com/