There havn't been devices with SFOS preinstalled for a long time (in fact, the Jolla 1 was the one and only). After that, there was only the Aqua Fish which was available only in India and the few devices re-branded as Jolla C (as community device) which were impossible to get a hold of.
After that, Jolla switched to selling only licenses for SFOS which you can install on certain Sony devices.

So to get a smartphone with SFOS, you'd have to get a supported Sony device (most likely an Xperia X, Model F5121 or 5122). Then, you'll need to buy a license from the Jolla Store (although it is only available inside the EU, you can simply use e.g. a VPN based there to purchase it) and flash it yourself.

Note that there are also "community versions" for quite some devices. Those are versions that community members build for the specific device and do not have any support from Jolla. If you want a quick and cheap way to test SFOS, they may be a good start. But please check the status of the version for your chosen device first as it may be outdated (the current version of SFOS is 2.2.0.29) or have severe bugs or limitations. Also, the community versions come with some features missing (text input prediction and Android support) since those are licensed by Jolla from third parties which do not allow to redistribute them.