The nuts that are welded into the floor have standard 5/16-18 (coarse) threads. Nuts in that size should be readily available at most any hardware store.
You'll need to clean any old paint, rust, etc. off the area of the floor to be welded. Also, strip any zinc finish off the nut or look for plain (un-plated) steel nuts. A bolt or piece of threaded rod screwed into the nut will help to hold it in place for welding. And using a brass bolt might be something to consider ... the weld won't stick to it if you accidentally get a little too close with the weld.
Rather than trying to weld the nut directly to the floor, you might find it easier to first weld the nut to a small chunk of thick sheetmetal. Drill a 5/16" hole in that piece of metal and put a bolt through it to hold the nut in place for welding. Then place that piece of metal on top of the floor with the nut sticking down through the hole (might have to open up the hole a little). Finally, weld that plate to the floor.