Syringes can be crafted by accessing the "Craft" section in the pause menu. Plants are needed to make the final product. Plants grow everywhere in the wilderness and are displayed on the HUD when you are close to them. The most common syringes are Medicine (2 x green leaf), Animal Repellent (2 x yellow leaf) and Enhanced Perception (x2 crimson leaf). There's also a skill called "Horticulture 101" that allows you to harvest two leaves from each plant instead of just one.