In some states/countries you can get ivermectin OTC. It’s a bunny safe mites-treatment, but it’s not sold specifically for rabbits OTC so dosage can be a bit of a problem. Do NOT give a pea-sized amount of horse ivermectin dewormer, as is sometimes suggested on the Internet. That is a huge dose for a rabbit.
Permethrin is available OTC and highly effective against mites, but it’s not considered totally rabbit safe. There are a few studies that point to it being rabbit safe, but those I’ve seen have been made on very few rabbits, like this one with only 4 rabbits:
http://www.researchgate.net/publication/286493942_Succesful_treatment_with_permethrin_for_fur_mite_infestation_in_pet_rabbits_Case_report
Then there are various herbal remedies that claim effectiveness against skin parasites, but many herbal compounds/essential oils are highly irritating and haven’t gone through proper testing so they might not be safe one bit.