Stim is the only medical item available to Cal in Star Wars Jedi: Fallen Order.
You can unlock improvements that increase the effectiveness of the Stim treatment, but the only way to increase the limit of these items is to find chests with containers. By default, BD-1 has only two Stims, but you will find one more on your first visit to Bogano, and upon returning you will receive a second one.
Bogano
Stim container 1. There is a descent down next to the place where the Praying Mantis has landed. It leads to a blocked passage. To open the way in, you will have to wait until Cal gets the Power Push ability. When this happens, return to the planet Bogano and gain access to the new Stim.
Stim container 2. In the underground shelter area, go to a circular room resembling a mine. Go over the wall and jump to the top. There is a large platform at the dead end, with which you can go down using the Force Pull ability. Again, it will not be available the first time you visit the planet.
Container with Stim 3. When you learn to run on the walls, from the “Mantis” go forward and to the left. This path leads to an area with three opponents. There are walls here along which you can run forward to find new enemies and go down into the pit, where a golden chest with stim is hidden.