Blazes can spawn naturally in the bounds of a nether fortress even without a spawner, but they don't spawn anywhere near as frequently as they would from the spawner. As long as you can create a valid spawning platform in the nether with the bounding box still defined, getting blazes to spawn is possible.
The only way you can get your hands on the desired items is by killing blazes. Unfortunately, this flying enemy only hangs out in the nether. So, before you can acquire blaze rods, you need to build a nether portal.