Finding old potato packages (downgrading libpng2)

[Graham Williams <Graham.Williams@cmis.csiro.au>]

I need to downgrade libpng2 from the current 1.0.5 in potato to the
older 1.0.3 (which I believe will fix a problem with pdflatex in
tetex-bin - Debian bug #49834).

Where do I find old version of deb packages?

Cheers,
Graham