You should have gotten your 1095-A no later than mid-February, but only if you purchased health insurance in 2017 through Healthcare.gov or your state’s Health Insurance Marketplace.
If you didn't receive your 1095-A or lost it, you can get a copy from your Marketplace online account or contact them directly for assistance. Here is the link for how to get a copy of your 1095-A from healthcare.gov: https://www.healthcare.gov/tax-form-1095/