.. Getting a bottle of Rocket Soda

You would need a voucher for rocket soda.

Are you so far in the story that you can go to the Slum?

There is a rocket soda voucher laying in the ground in front of the bioscanner. ( Go to Clubs Alley and the Narrow Way is at the far right, after the club. )

Use it on the rocket soda dispenser to get a bottle.