I'm not sure where you are located. In the US you can find it at Home Depot, Lowes, or any decent garden center. Also, re-reading my post I spotted a typo.
Spider mites have a 10 - 15 day hatch rate. You should apply the oil for 2 days straight and then wait for a week. Then spray every other day through day 14.
should have read:

Spider mites have a 10 - 15 day hatch rate. You should apply the oil for 2 days straight and then wait for a week. Then spray every other day on days 10 through day 14.

Also note that you do not want to be spraying oil on your buds so once they go more than a few weeks into flower your options will be more limited. You can also get some "hot shot no-pest" repellent strips which should help keep them at bay after you complete your neim oil treatment. Run a search Hot Shot No-Pest and you will find them.

Good luck!