When you have zero Slow-Mo left you will notice that the icon to activate Slow-Mo will no longer display the amount that you have left and this number will change to a Plus (+) symbol. Simply tap on the Slow-Mo button and you will be shown a pop-up menu which will allow you to purchase more Slow-Mo for in-game currency.
Alternatively, you can purchase more Slow-Mo from the main menu by tapping on the (+) Plus button located in the top-righthand corner of the main menu.
Read also How do I activate Slow-Mo (Slow Motion)?
If you do not have enough in-game currency, you can buy more. Read: How do I purchase Special Offers, Gold, Cash or Consumables? or How do I make a purchase?