Have you considered offering a print on demand service? If I didn't have my own 3d printers I'd be more interested in sourcing models myself and paying for time/filament/power/pre and post processing. Just make sure the models you advertise specifically have a compatible creative commons license and maybe toss them a few bucks if they're popular on your store.
As for getting out there Craigslist and Facebook I would think are the most straightforward ways to advertise for free. Instagram could be useful as well.