lftp will do this for you. To grab a whole source directory use this
Code:
lftp -c "open https://mirrors.slackware.com/slackware/slackware64-current/source/a ; mirror cryptsetup"
Or this can work too, but will download some unnecssary index files and others, but it works
Code:
wget -r -np -nd -l1 https://mirrors.slackware.com/slackware/slackware64-current/source/a/cryptsetup/
Another scenario you may run into is a SlackBuild with multiple source files that need to be downloaded. For that you can do
Code:
source <projectname>.info
wget $DOWNLOAD