As you mentioned, we have TurboTax Download/Desktop (for Windows) for 2017. You can use this to NETFILE or print and mail your 2017 return. You can find it under the 2017 tab here: https://turbotax.intuit.ca/tax/software/prior-year-taxes
We no longer have any TurboTax products available for tax years 2016 or earlier. However, you can still download the forms for those years from the Canada Revenue Agency (CRA) here: https://www.canada.ca/en/revenue-agency/services/forms-publications/tax-packages-years.html
NETFILE is no longer available for 2016 or earlier, so you would need to print and mail your returns.