They were originally hosted on the games forum (which was taken down due to the new GDPR law in europe.)
I moved the community hub to discord since then. One member of the community created a spreadsheet which contains the information of all available custom mapfiles (with download links) and posted it on discord. Here is the link to this spreadsheet:
https://docs.google.com/spreadsheets/d/1Al-EOqTRAZvPsHNv4T9bRiQCdTQvrTu8yyQulcJT...