Getting a specific old version of a package as a .sty file

The Problem

I’m trying to submit a manuscript to a journal hosted on mc.manuscriptcentral.com, which, as far as I can tell, use a very old TexLive distribution. Naturally, this is out of my control. It’s so old that I’m having trouble getting my paper to compile with siunitx: I don’t remember having this issue with previous manuscripts now, but using it reveals lots of errors along the lines of

! Undefined control sequence.
<argument> tl_length:N 
                        l_siunitx_tmpa_tl 
l.272 ...sine pulse with a SI{500}{microsecond}
                                                   nutation frequency was ch...
The control sequence at the end of the top line
of your error message was never def'ed. If you have
misspelled it (e.g., `hobx'), type `I' and the correct
spelling (e.g., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

What I don’t quite understand is that tl_length appears to have been a macro in the experimental package latex-3 in l3-kernel that was removed from the distribution at some point in the past: on my local machine, if I do cat /usr/local/texlive/2016/texmf-dist/doc/latex/l3kernel/l3obsolete.txt the relevant bit tells me so — it was removed in January 2013.

So, I naïvely tried to proceed by uploading siunitx.sty from my texlive distribution to the journal’s online web platform. This changes the error, at least, to something more human-friendly:

) (./siunitx.sty

LaTeX Warning: You have requested, on input line 16, version
               `2015/09/11' of package expl3,
               but only version
               `2013/03/14 v4469 L3 Experimental code bundle wrapper'
               is available.

Package: siunitx 2017/03/23 v2.7f A comprehensive (SI) units package

! Package siunitx Error: Support package expl3 too old.

See the siunitx package documentation for explanation.
Type  H <return>  for immediate help.

The Question

Ideally, I’d therefore like to be able to download a version of siunitx.sty from texlive ~2013 and upload it separately to the publishers’ website.

Is there a simple way to easily find this, and download just this file using standard package management tools and free websites? (Moreover, any other insights are welcome!)