There is a big difference between GPLv2 and GPLv3 here.
When distributing GPLv2-licensed programs/libraries (in executable form), you must
provide a download link to the same server as where the executables are downloaded from (to satisfy the clause below), or
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, [...].
provide a written offer to deliver the source code to the asker. This option seems to be intended for use with physical products that contain embedded software, but might work for downloadable software as well.
Compared to GPLv2, you are now officially allowed to separate the locations where the binaries and the source can be obtained from, but you are still responsible for providing access to the source. This means that you can link to the original site to get the source from, but you need to have a fall-back scenario in place for when that site goes down.
Additionally, the 'written offer' option is only available if the software is part of a physical product.