In Instrumentor Pro you can use any third-party AU ou AUi with your MIDI USB Keyboard, combine for example Output Effects or Instruments (AU, AUi) with Klevgränd Produkter AB effects and instruments work with any AU or AUi installed on your system. Compatible with macOS 10.13 and up. Find at macOS Apple App Store.
The App is specially designed to use in live performances if you want the AU and AUi for performance and don’t want to use a DAW.
You can get free instrument and effects here :
There is also an awesome article with a huge list of free plugins that also would help to get best sounds in your projects and performances you can read the article here:
You can also combine with Output Instruments and specially with Output Arcade check this page for more information : https://output.com/products#instruments
FREE PLUGINS FX VST3,AU,AUv3 MoStudios Pack
Make your very own MIDI instruments and play it without the need of an exs file for macOS