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!
bigsby Reviewed by bigsby on . SOS Help MItes plz HELP Hi i have some old bitcthes autoflower...ak48 that finaly go to the flower stage after turn to 12/12 till then only veg no sign of buds..... now after 35 days flowering has buds with fresh pistils that waiting to get fat.. Together in the draw are some babys with thes bitches..... And i saw mites....i get mites phobia of how easy is to inhale this bastards.... So i just cut and remove some big leaves with mites on it... What else to do.... Is it possible to fuck my babys......??? Rating: 5