Do a google search on "zip-a-tone." Here's a
tutorial I found on trying to make them in Photoshop. It was a product cartoonists used, plastic sheets with the dots, sticky on one side. You'd cut them out with an exacto and place them on the artwork. Thing was, I don't think the professionals ever used Zip-a-tone sheets. And that won't help if you want to make them in color.
I once needed to make a flyer with b&w photos and I was going to make lots of copies on an old xerox machine. The photos wouldn't have any tonal values after being xeroxed, so I printed the flyer on my inkjet in Corel, and made some sort of setting for preparing for postscript or something in there, and it converted everything to the dot pattern for me. Wish I remember how i did it. I don't have Coreldraw installed anymore and, sorry, I don't remember how I did that exactly. But I did do it, and I'm sure it would have worked for color.
You might be able to get that effect just printing to a laser printer.
What are you up to, anyway?
