There are 5 throwable upgrades.
Upgrades can only be done in First Aid Tents via the computer inside. The first one you encounter is on the farm.
There are 2 types of upgrades (Crafting and Harper) The throwables are the Crafting Upgrades.
To be able to do an upgrade you will need to find enough "Red Weed Extract" which is generously placed around the environment.
The 5 throwables needed for the achievement are:
1: Water Jar (which will cost 25 Red Weed Extract)
2: Shocker (which will cost 30 Red Weed Extract)
3: Smoke (which will cost 40 Red Weed Extract)
4: Flashbang (which will cost 50 Red Weed Extract)
5: Rubber Explosive (which will cost 60 Red Weed Extract)
You have no choice to get these in order from 1 to 5. Only the next one in line can be purchased.
You will get your first throwable upgrade at the farm. It is needed to proceed to the next section. There is just enough "Red Weed Extract" on the farm to get the "Water Jar". They won't appear until you have been through the Meat Plant.
It is unlikely that you will get both this achievement and the "Strong and Northern - Fully Upgrade Harper" in the same playthrough. I scoured each area thoroughly and had all the throwables, but only 3 of the Harper Upgrades by the end of my 1st game. Luckily there is a New Game+ that lets you start with everything you have at the end of your 1st game. I got enough "Red Week Extract" in my New Game+ before I got to the caves section to complete both achievements.