Custom made - yes. Flat glass yes - most glass companies will cut what you need from dimensions or a pattern. Curved glass is a completely different challenge and glass companies that I know of don't venture into it.
Yes, and no. I chopped my 55 Olds back in 84, and have done many since then. The flat side glass (door, quarter, and vent) can be cut from new flat glass, or the old ones trimmed down (not recommended if they are de-laminating, or damaged) The front windshield, which is laminated, like the side glass, can be cut down as well, though curved glass is difficult to do. There are quite a few methods out there for doing it, I can't comment on which ones are best, I do it MY way. Another problem with front curved glass, is that a lot of top choppers don't take into account how trimming and laying curved glass down to fit, will affect the windshield opening. Some make it impossible to cut the original glass and have it fit. Rear windshields: originals are mostly 'tempered' glass, and CAN NOT be cut. You can either change it out to an easier windshield to fit, or cut the entire sheet metal surround out, and lay it down to meet the roof. Some move it forward, and extend the area between glass and trunk. In any case, it involves serious sheet metal surgery. Aftermarket rear windshields are often laminated, so if you buy replacement ones, you can cut them.
Dumb pet peeve of mine, I know .Only the front glass is a windshield. The rear is the backlight or, simply, the rear window. Back to our regular programming. This has been a PSA from the local pedant
Some people cut the glass and chop the car to match it. Makes sense based on @chopolds comment above.
I have seen 3 or 4 chopped projects on the local CL for cheap and figured they were giving up on them. skeered me away on one because the chop quality and metal work we're sub par let say. I like a challenge but it can be a can of worms following behind someone else's work and ideas.
A friend has done several ,he takes the whole windshield and buries the bottom into the dash. He says its easier than it sounds. They look good to me.