You can either buy currently made headlight assemblies, of which there are lots of models, or you can shop at swap meets or Ebay for a cool set of old headlights and use those. Back in the 50's and 60's there were a bunch of companies like Guide, Dietz,KingBee, etc that were cranking out different kinds of headlight assemblies so that people could update their old cars to sealed beam headlights. These have a lot more charm in my opinion than some of the current stuff, and are sought after to a large degree, so some bring big money.
Anyway, most of the lights, either reproduction or old stuff, have a common stud mounting system that either allows you to mount them to a dished mount or a flat topped mount............adapters usually come with the new ones to fit both style bases. Sometimes you have to get a little creative and modify the bases a little for a tight fit so they don't move around on you, but generally it is plug and play.
Also, if you pickup an old set that were sealed beam and they were 6 volt, single headlight 12 volt bulbs are exactly the same size so they go right in. I think the law used to be that the headlights had to be so many inches high (something like 24") but I have never seen that law enforced, plus, many of the new cars being built are lower than 24 inches high, so I wouldn't sweat it.
Don