I wonder if there is a SharePoint 2019 Foundation, I know that it's not the case for SharePoint 2016.
Not a free version, but a trial version in Azure.
No free or foundation version of the SharePoint server 2019. Only aviable option is use the trail version of SharePoint 2019 standard or enterprise.
You need a server OS in order to install it or you can use the Azure template as other mentioned.
You can download the bits of SharePoint Server 2019 from here.