I don't know if you're still trying to find them, but there's a way to export your fonts from the font management screen (you don't have to know where they're saved). In Settings > Appearance > Font Management, under "groups", right-click either "All Fonts", "Personal Fonts" or "System Fonts" depending on which group the font you're looking for is listed under and pick "export" from the drop-down list. Select a location to export to and give it a name, then view the contents of the resulting archived file. Your font should be there if it was in the list of fonts in Font Manager. Then, just extract to wherever you want.

Note, you may have to rename the font file to something .ttf or .otf after you extract it. Some of mine have exported as woff or woff2 and won't open until they're renamed .ttf or .otf but function as expected after doing so.

Kubuntu 22.04.1
Plasma 5.25.5
KDE Frameworks 5.98.0
Qt Version 5.15.3