You're half right... and half wrong. There is an Alien Blaster at the crashed spaceship site. There is also a bunch of ammo lying around there. If, while you are trying to pick up alien ammo, the ammo falls below the ground surface, wait a sec, and it will bob back up.
There is also a random event, triggered, at various points throughout the wasteland. This is known as the Firelance event.
Spoiler An alien spaceship will explode nearby in the air, and ammo will rain down on you, as well as a unique alien weapon that is more powerful that the Alien Blaster, by far.
Some playthroughs, I have never received the Firelance event. Some plays, I have died, because some raider found the weapon before me, and killed me with it --->(I play DiD only.) One play, the Firelance fell on the top of the Super Duper, and I never got it. One play, the alien spaceship exploded ON me. The amount of radiation my character received, for about ten seconds, was amazing. I had to build a ladder, out of single 5mm cases to get the weapon on top of a raised bunker, but I got it.
So, go back. I use the English bobby routine, back and forth, grid pattern. If you have Dogmeat, he will find it for you.