How can you obtain a prescription for invermectin?

Cannot do it here, as this is a public education site and we neither diagnose or prescribe. Contact your primary care doctor and explore whether this is a drug needed by you.

Dangerous shortcut. Drugs are to be used for diagnosed conditions or symptoms when they are believed to provide relief or cure for in a clear medical context. Asking for a drug will sound suspicious to any physician that has no knowledge of your situation. I agree that your best starting point will be your physician, who might be able to understand your request in your personal context.