I have seen some photos of Apollyon, Despotron and even Mightron holding the blue/purple laser rifle Megatron used to shoot at Optimus in the film. However, I don't know where people got that gun, because I thought it was an accessory of Apollyon or Despotron. But I found that none of them have that gun as an accessory, so from where do people get that gun?

 

You get one with Apollyon.

 

It comes with MP5 who should be quite cheap nowadays considering the other options available.

 

The one in the photo you posted came with Apollyon. They can be found loose on eBay from time to time.

I also have one from MP-05. If you want it, you can have it for free if you pay for shipping.

 

Try this: MP-5 TRANSFORMERS MASTERPIECE MEGATRON ENERGON LASER BLASTER GUN TAKARA

 

or this: MASTERPIECE MEGATRON ACCESSORY SET KREMZEEK SWORD GUN SOLD AS IS SHIP WORLDWIDE!

 

Thanks Ruizu, but I am embarrassed to say; I do own an MP 05. However, in my own defence I put it back in the box and sealed it again. The reasons is that I found MP 05 quite disappointing. I just reopened the box and found the gun.

Thanks guys for being so helpful!