Control depends on the kind of spider mite you have. If it's the almost invisible, web spinning kind that
causes plant leaves to yellow or brown and fall off, then the best miticide is Avid 0.15 EC http://growersupply.com/av8ozmi.html
It's expensive and systemic so your milkweed might not be suitable to feed monarch caterpillars for a month or more, but extremely effective against spider mites.
Maybe you have the larger predatory spider mites. If that is the case, these can be controlled using Ortho Malathion Plus available at Walmart, Home Depot, etc. Use the dilution rate recommended on the label for mites. Malathion is not systemic and treated plants are safe to feed monarch caterpillars about a week after spraying.