I need to have a dispenser able to dispense an infinite amount of Splash Potion of Harming II, and from what I can see the best way is getting a -1 stack of the item, but I can't figure out how.
I don't know about giving yourself -1 of an item but to get an infinite dispenser you can use the replaceitem command. It is entered like
replaceitem block [x] [y] [z] [slot] [item] [amount] [data] [dataTag]
If you need more help you should just be able to make a quick google search for replace item command in Minecraft in 1.8.9 or Infinite dispenser in 1.8.9
Otherwise this post is good: How can I make a dispenser that has unlimited arrows?