The icon set in C:\ProgramData\GPSoftware\Directory Opus\Icons\DOpus9.dis may be a good place to start, if you just need an idea of what each icon is.
You can also find icon grids with the sizes for your current DPI under /dopuslocaldata/Icon Cache (paste that into the path field and push return to go to the appropriate folder for your machine).
The Toolbar Icons page in Preferences can save an XML template with all current icon names, and examples of how to handle DPI:
/dopuslocaldata/Icon Cache contains no dopus 13 icon set for me. I got a bunch of other icons I have downloaded from the internet but no Dopus 13. This is strange because I can still enable the Internal icon set in the preferences.
Ah, my mistake. They'll only be there if a special set like the "Small Icon Set (Flat)" is resizing the internal icons to additional sizes.
Another way is to go to Prefs / Toolbars / Icons, select the internal set, then use Menu > Save Icon Set Template while holding Ctrl + Shift. That'll output a .dis file which includes the XML and icons (only for the current DPI, not all DPIs).
(Also note that if you only want to add or modify some icons, your set doesn't need to include all the default ones. You'd only want to do this if you wanted to combine some of the existing icons into new ones.)