First and foremost, flat screens can be available in many different types of monitors, including crt. I used to have a flatscreeen Ching fu or whatever brand crt.
For LCD:
You can get a scan line generator. It's a piece of hardware you plug inline with vga that adds scan lines. The good is that everything from the computer has scan lines. The bad is that it only adds scanlines and doesn't simulate any other features of crt. I probably have an extra I'd sell if you messaged me.
There is software shaders. These do a decent job of simulating scan lines, bloom, etc of a crt. The bad is that it only works with mame and some front ends.