It is easiest if you use lights that were designed to use a standard 7 inch bulb. That way you can go to any AutoZone and pick up standard or halogen bulbs that were made for cars like 56 Chevys and bolt them right in place. Some of those will have 6 volt bulbs in them when you find them, but the 12 volt 7 inch ones are a direct interchange.
There were a lot of companies making aftermarket headlights back in the day. The most common ones are Guide, KingBee, Dietz, BLC, and others. People wanted to replace their inefficient lights that came stock on model a's and other cars with the new, brighter ones so lots of companies made lights and conversion kits to fill that need. Also, many cars and trucks came with these aftermarket lights from the factory.
As was mentioned, you need to find a set that have the look you want. Before the rat rod craze most people just ran the traditional bolt on lights from the companies mentioned above, and bought them from places like JC Whitney. But the rat rodders started using uncommonly used ones, and generally, the bigger the better.
If you find a pair of Guide lights that need some parts, OTB carries every part for them, down to the retainers and trim rings. I bought a pair of new trim rings for the Guides on my 27 from OTB, and they were perfect reproductions and high quality. Ebay is also a great source for headlights, and at one time I had 15 sets that I bought at various times. I thought that was a ridiculous number to own, then BIG-JIM posted that he had 300!

He is my new hero.
Don