The Jetpack mod is only available via crafting at a Power Armor Station, and requires "Science! Rank 4".
You can still obtain one without that perk, according to Fallout wiki:
T-60 variant can be obtained as a reward for completing the quest A New Dawn.
However, Jetpack mods are craftable only for T-45, T-51, T-60 and X-01 series.
So I don't ...