Quoted:
Kelthane, if you have any. But red spider mites develop resistance to it so it only is effective once per season.
Be DAMN careful with soaps and oils as these are TOXIC to tomatoes. Much more toxic than Kelthane.
Keith J speaks the truth. Red spider mites are a pest in peanuts as well. When it is hot and dry, they can erupt in your field, or garden. Walking through the field, or running garden tractors or tillers through the garden or field spreads them too. Kelthane lost its label in peanuts, but it used to work well. It is actually less toxic than the product that replaced it. It is still labeled for tomatoes as far as I know. It is safe enough to use one day, and eat the tomatoes 24 hours later. So...
If you can get a little Kelthane, use it. If not, then pray for rainy, cooler weather, and they will die off on their own. I am not sure about other products that you might find at a garden center as I never used them for mites. As a precaution, NEVER cut the grass or ditchbanks near your garden while it is dry and hot. You will just spread these mites from those areas to your garden. Stay out of those areas as well, because you will transfer them into the garden.