No idea about windows but this is for Linux a free OS such as redhat, ubuntu, freebsd
https://www.clamav.net/doc/mirrors
Ubuntu server - https://www.ubuntu.com/server/get-ubuntu/download
How to get started with ubuntu server with ClamWin - https://www.linuxtopia.org/online_books/system_administration_books/ubuntu_starter_guide/ch07s02.html#installclamav
If this was me i would install ClamWin on a vmware image like virtual box install Ubuntu server x86/32bit and take everything like
main.cvd
daily.cld
bytecode.cvd
from /home/users/clamavdb/public_html into a http server under windows and point the virus database updates to my network ip address
192.168.1.54 and see if it updates from there.
Hope this helps for you.
Fran