You might be able to get a "high street printers" to help.... I think most of them will do/A1/A2 black and white as posters pretty cheap (light stock), then you could fold it down for your mock up, I imagine you'd need to impose it yourself... might be worth a try.
For small runs you need a digital printer but A3 is normally the largest they print, so that wouldn't help for a broadsheets unless you wanted to get it printed as single sheets then stick it all together
don't think it will be cost effective for anyone to do mate.
If we do a broadsheet mockup , we would make a n 8 page dummy by stapling 4 sheets of A2 very thin layout paper 70- 80 gsm in the middle then creasing. Then spray mount a3 inside, trim at centre first and leave ALL the out edges oversize. then when finished close and fold to A3 size then trim all three edges flush. Dont trim and mount individual pages.