Think of voltage as the pressure like in water pressure. To get 300W of light out of a 12v circuit, you need a fire hose. Amperage is like the volume. 300/12= 25A. At 120volts, and 300W, you'd only have 300/120=2.5A, or 10x less volume (and you could use smaller wires). Both would use 300W, but one would be doing it by high volume, and the other with high presssure. Because you can't push 25A through the 120vac lamp (you'd get only about 1/10th, not exactly since the resistance changes with temperature), while it probably would light, it might only barely glow and you'd not get anywhere near the illumination out of it.
If they no longer make those bulbs, you'll have to change the fixture. Have you looked on-line or at a well stocked electrical supply store? It's not likely to be something you can find at someplace like HD or Lowes.