Get a Hydrobrick Maxx or OG brick and use any 18mm injector. @VGOODIEZ has Bricks for 20% off currently and a number of ballvapes as well.
If you want a handheld, you'd want the OG--I'm not sure a Runt would work well, but it may. I'd recommend the Maxx, though, since it's easier to see what...