Tarballs are usually provided by upstream.
But sometimes tarball creation by gentoo developers is necessary.
If upstream project is alive it is better to join it and to create necessary tarballs.
Or to ask authors to create some release.
If this is not possible (for dead projects or uncooperative (too principial for example) upstream authors), then
it is necessary to create and store tarballs by ourselves.
There is no central storage of gentoo-rolled tarballs in gentoo,
see https://devmanual.gentoo.org/general-concepts/mirrors/#suitable-download-hosts
for hosting tarballs with for Developer Webspace feature