Persimilis is the main predator of spider mites and is the only one that will go directly into the webbing to eat them. They do best at a humidity of at least 60%, or you can help by misting your plants twice daily to keep the environment humid (pest spider mites love dry conditions). It may be necessary to reintroduce them after 7-10 days if you're dealing with a heavy infestation, or if conditions are sub-optimal.
Helpful Hint: One thing to be sure of before introducing persimilis (or any other type of beneficial bug) is that you have no pesticide residues on your plants.
Mighty Mites (P. persimilis):
1,000 Mites (22750): $56.00 on vermiculite (for areas ~500 square feet)
2,000 Mites (22751): $76.00 on vermiculite - smaller "hot spot" vial (for ~500-1000 sq. feet)