I have a decorative rotary phone on a shelf below some pictures and mementos of my wife's grandfather. When he was alive, he loved to tell stories, and one day i picked up the phone, put it to my ear and thought "what if i could dial up Marvin and hear one of his stories?" Now i can, and what's more, it led to an art installation in a local coffee shop.
This is the easy part and the hard part. You can find them in many places, including eBay or occasionally still thrift shops. But finding one you love could be harder. The parts of the electronics you'll use in this project are not that complicated, and very resilient so for the most part the way it looks is more important than whether it actually works. Obviously if it works as a phone you know all the parts you need for the project will work too, though.
take it apart and inspect it
This is the interesting part. Two phones which have the same function, and look almost exactly the same on the outside can be engineered completely differently inside. Some have discrete components visible, some have everything hidden in a metal box or encased in hardened plastic. There are interesting techniques for relieving strain on wires, making connections, etc. There may even be hand-written notes or schematics on pieces of paper inside. Take some time to appreciate the engineering, artistry, and manufacturing prowess before you start thinking about it too practically in terms of the project. Unscrew the handset mouthpiece/earpiece covers too, and look at how the mic/speaker are connected. Take pictures of everything. They can come in handy later.
identify the components/connections you need
Find the following wires/connections.
The rotary dial and the (usually 4) wires coming out of it.
The earpiece and ground/common connections from the handset (sometimes you can tell by the color of wires at both ends, sometimes you might need to use a continuity tester).
Two connections from the hook switch that are connected when off-hook and not when on-hook. I usually look for screw-terminals that connect to these and don't seem to connect to too many other electronics inside the phone.