I am sorry to hear your cats all have ear mites. My name is Rebecca. I am a veterinarian and will do my best to help.
Are you sure they have ear mites, and not an ear infection? If you treat with something for mites and this is an infection, you will be disappointed and frustrated about spending money on something that did not work. Ear mites are overdiagnosed as the cause of ear infections; most people assume an ear infection is mites but often that is not the case.
You ask two questions and they have different answers. The best treatment is to use Tresaderm in my opinion. Second best is to give each cat oral ivermectin. Tresaderm is available online but you have to have a prescription from a veterinarian. When you call the online pharmacy, you give your vet's information and they contact your vet to see if that is OK.
Your second question is what can you buy online. There are many products available without a prescription but they contain pyrethrins, which can be toxic in cats and are not as effective as prescription medication.
My best recommendation is to take one cat to a vet and confirm this is mites and not something else. Most (but not all) veterinarians will send you enough medication to treat all the cats if you have 10, and know their weights. 2 or 3 bottles of Tresaderm should treat all the cats. Alternatively, a dose of ivermectin could be calculated for each cat (it is very concentrated and easy to overdose) and sent home with you.
You can buy ivermectin (only get the brand Ivomec) in feed stores and online, but you would have to buy the whole 50 cc bottle, and you only need about 1 to 2 cc to treat 10 cats. The dose is .1 cc (you will need a syringe; this is less than 1 drop and too much is dangerous. Ivomec is a cattle wormer and is very concentrated and easy to overdose) by mouth per 8 pounds of body weight; .2 cc to a 16 pound cat. You repeat this in about 10 days.
Let me know if this is helpful and what other questions you have. Rebecca