You can buy it in concentrate on Amazon and dilute to the appropriate level. A manual application typically does not last for 18 months. The manufacturer-based applications for insect repellent clothing CAN last for 18 months.

It IS a wonderfully effective repellent not just for mosquitos, but also ticks, fleas, and lice.

See https://sectionhiker.com/treating-your-clothes-with-permethrin/ for how to use it.

Now, word of warning: Many people do get reactions to it. I generally don't, but I still would NOT treat garments or underclothing with it.