No responses? Well, I'm no expert, but my method has been to take a ream of "good" paper to work and use the big behemoth copier/printer. Fused toner is much more water-resistant than most inkjet inks - not that fountain pen ink would smear them, but rain, spills, etc. could.
The other option I can think of is looking around for a small print shop, if they haven't all gone out of business. They should be able to set up whatever you'd like, in a range of techniques. I remember having some invitations printed out on store-bought note cards; this was years ago, but the price was minimal for the quality received. It's worth looking around.
Barring that, a large copy/print shop should be able to do reams at a time, though I couldn't speak for price. I'd hope they offer a bulk discount...
Or perhaps an XXXXF nib and a drafting rule

?
Best,
Jon