You can buy the Sentinel blueprints for 100,000 credits apiece. I decided to go with the Dethcube. The thing pretty much levels itself up. Once equipped, just continue playing the game and it will be leveled up in no time.
Side-note; The Dethcube's weapon is considered a machine gun, so if you haven't yet leveled up a machine gun to level 30. or want to use this one as part of your five, do it.
Update: They have updated the game and the Dethcube's weapon will no longer count towards the Rifle Proficiency achievement.