How to download macports packages separately?
You can use port fetch rdepof:$pkgname and not installed to trigger only downloading source code. Given that you want to use download accelerators, that probably doesn't do what you want.

If you just want to get the URLs for downloading, see port distfiles rdepof:$pkgname and not installed and parse its output. Note that you may have to deal with ports fetching source code from Git/Subversion/etc.

You can use a script I wrote a while ago as a starting point; it already parses the output correctly and initiates the download (using curl), however it downloads every URL in the mirror list, not only one of them. The script is at https://trac.macports.org/browser/contrib/port-checkmirrors/port-checkmirrors.