As it is after the 15th of the month, you are shopping for insurance for NOVEMBER. It's too late to qualify for October, unless you qualify for Medicaid; then they might be able to retroactively date your insurance. Something we found out the hard way, too.
ETA: Rules for ACA vs Medicaid are contradictory: ACA uses annual income, but Medicaid looks at monthly income. When we applied because DH was between jobs, I expected to be eligible for ACA subsidies based on annual income, but the monthly (temporarily) low income shunted us to Medicaid instead. YMMV, based on your state's Medicaid status (expanded or not) and rules.