Buy it at the space station. The specific operation steps are as follows:

1. Open the task manager and click the "Special Test" task.

2. Find antimatter in energy factories and antimatter factories.

3. After finding the compound factory I of Tyre Refined Products, you can see the "antimatter unit" through the list in the lower right corner.

Remember to dock the spacecraft at the mission point as shown in the following figure, and you will be given a space station after the mission is completed.

After completion, we can find the space station presented to us in the assets.

6. Find Thunderbolt in the space station and click Buy.