For your original question, you can find the rifts before taking Juice but it's much harder. There's always a blue pentagon circle thing where a rift will be. (They've actually been in the base game long before this quest so you might have seen some in previous playthroughs.)
As for your supply of juice, you can bring Dude the ingredients and he'll make you some or you can ask him for the recipe and make it yourself. Making it yourself takes more ingredients though as a trade off for being able to do it on the go. You can also set up a supply chain to get Dude his ingredients and he'll also sell you Juice for a decent price. I seem to remember it was cheaper than a train ride, so a pretty good deal.
