You can get a list of themes available via the repositories through Synaptic Package Manager or through the command line via aptitude :

$ aptitude search plymouth-theme p lubuntu-plymouth-theme - plymouth theme for Lubuntu p plymouth-theme-fade-in - graphical boot animation and logger - fade-in theme p plymouth-theme-glow - graphical boot animation and logger - glow theme i plymouth-theme-kubuntu-logo - graphical boot animation and logger - kubuntu-logo theme p plymouth-theme-sabily - plymouth theme for Sabily p plymouth-theme-script - graphical boot animation and logger - script theme p plymouth-theme-solar - graphical boot animation and logger - solar theme p plymouth-theme-spinfinity - graphical boot animation and logger - spinfinity theme p plymouth-theme-text - graphical boot animation and logger - text theme c plymouth-theme-ubuntu-logo - graphical boot animation and logger - ubuntu-logo theme i plymouth-theme-ubuntu-text - graphical boot animation and logger - ubuntu-logo theme p plymouth-theme-ubuntustudio - Ubuntu Studio Plymouth theme p xubuntu-plymouth-theme - Plymouth theme for Xubuntu

You can then install the resulting packages via Synaptic or apt as normal:

$ sudo apt install plymouth-theme-solar

If you're looking for something not in the repositories, UbuntuGeek has an excellent tutorial on how to install and create your own custom Plymouth themes.