I can't find a second generation plasma rifle, where is the best guns shop withe best selection?

It seems to be level based on when the mk2 and ultra weapons show up, I wouldn't worry about it play enough and you will be swimming in weapons you with most likely sell or breakdown for repair parts. You can check the random vending machines speckled across the worlds or check the shops on ground breaker eventually one will pop up for you.

You should just come across it naturally .What I mean is think the game has a good pacing in that you will run into ever better weapons as your progress as you need them. You probably just need to get farther in the game. Best yet, you will probably get it off someone you killed, so it will be free.