Home Depot has the strips. You'll find them in the garden section by all the other bug sprays. They work for awhile, like several grows before the mites become immune to them. After that, AVID is your best bet. Five drops of AVID to a quart of water is all you'll need. I wouldn't recommend AVID for flowering plants though ... use it during veg. Also, after your harvest, be sure to clean everything well, spray AVID in all the nooks and crannies. Mites are persistant little bastards that will lay dormant and keep coming back until you kill all the adults and their eggs.
AVID is expensive, but some hydro stores sell small vials of it. Like I said, its very potent, so you don't need much. Again ... just 5 drops to a quart of water in a spray bottle and the mites are gone. Be sure to spray the undersides of the leaves.
Vi