Sorry----software for 2018 is no longer available nor is it supported by TurboTax. Even if you could find it on eBay, etc. you would not be able to get updates or support; it would not work.
What are you trying to do? It is too late to amend the returns for that year and get a bigger refund. If there is some other reason for making changes to 2018 returns, you should seek local paid tax help.