MacSim

An older version of MacSim can be downloaded from here.
It will only work up to around OSX 10.13 or so and is completely unsupported.
It is also unsigned so the first time you run it you will have to approve opening it by visiting the Privacy & Security panel of the System Settings.

Free Licence for Rondo

Please note that Rondo will not work on any MacOS greater than 10.14 and is completely unsupported.

To license Rondo please enter the following info into the corresponding fields of the licensing dialog box. This can be found under the ‘Rondo’ Menu of the App:

Name: Rondo Free
Email: rondofree@fracturedsoftware.com
License No: 860T23-GXRLF-QRPED-71CYP-NC9N9-XQ65

The text must be entered exactly, as the license code is based on the name and email address. Best to copy and paste, avoiding extra spaces at each end.

MIDI files on the net

www.classicalarchives.com
An excellent source of classical MIDI files available both free and by subscription

www.mutopiaproject.org
A large selection of out of copyright music available as MIDI. Mostly classical.

Or just Google for your favorite!

Don’t forget to add MIDI to your search.