With Super Missiles, you can reach lower Brinstar, where you will find the Spazer Beam.

Go to Lower Brinstar

From where you got the the Super Missiles, go to the left and go through the door there. Go to the left and stand near the door on the left, and shoot it with a Super Missile. Go through.

Jump onto the ledge, then crouch and shoot a Super Missile at the base of the wall to open a hole. Roll through the tunnel and use bombs at the end to break through.

Go to the left to reach the bottom-left corner of the room, where you got a Missile Tank earlier. Go to the right and get on the ledge with the tunnel. Drop a bomb to launch yourself up to the tunnel, then roll through it. Shoot the door with a Super Missile and go through.

Go to the right until there is a pipe on your left and a pipe in the upper-right corner. If you wall jump off of the wall on the right, you can get up into that upper pipe, and roll through it to get more missiles.

Go to the bottom-right corner of the room and shoot the light above the gate to open it. Go to the right and go through the door.

Go to the right, then jump up on the ledge and run to the right by holding the Dash button as you go. This is the B button by default. You will run across some pit blocks to reach the rightmost side of the room. Shoot a Super Missile at the door to open it. Go through.

Get the Spazer Beam

Make your way down to the bottom of this room. When you reach what seems to be a dead end, go slightly to the right of where you landed, then shoot downward twice. Ignore the block to the right of where you landed, because if you destroy it, a Beetom will come out and latch onto you. It can be killed with bombs, or you can make it get off of you by pressing buttons quickly.

At the bottom of the hole that you opened, go to the left to fall through to the lower area. Keep going down until you reach two doors at the bottom. You can shoot the left door with a Super Missile to reach an energy recharger. Then exit the energy recharger room and go through the door on the right.

Hold the Dash button to run to the right and land on the platform in the middle of the water, then shoot the Skrees on the ceiling, but beware of the debris that falls from them. Go to the right and go through the door.

This room has Yapping Maws that will try to grab you and pull you down, so keep your distance. On the leftmost side of this room, shoot the ceiling above the entry door to open a hole in the upper-left of the ceiling. You can use wall jumping to go up through that hole (if you aren't able to do it, you can reach the upper area later after you get an upgrade). After you wall jump up through the hole and reach the ledge at the top, bomb the first block, then shoot the second one to avoid bombing open a hidden hole in the floor that will drop you directly onto a Yapping Maw. Go to the right and shoot the door with a Super Missile. Go through it to find a Chozo Statue holding the Spazer Beam. Take it. You will now shoot three beams in a spread. You can turn this on and off from the pause screen if you press R to view your equipment.

Leave the Chozo Statue room and go to the left through the tunnel, then down to the area with the Yapping Maws. Go to the right through several rooms and you will reach an elevator. Go down to return to Norfair.