I concur with my fellow Goldfarb (no relation, there are just lots of Goldfarbs into photography!) and I think that conventional b/w films are probably the way to go. And yes, the printing is where any tones (gray vs. brown, etc.) are controlled. But for poster size prints, the main issue that there's simply no getting around is that you really need to start with a good negative.(posted 8718 days ago)The real question for Jill is what kind of camera she's using. If it's a medium-format camera, good old Tri-X 400 developed in nearly any developer will enlarge to poster size without too much grain. However, if she wants to blow up a 35mm negative to that size, slower films like Kodak Plus-X or T-Max 100, Ilford FP4 or Delta 100, or Agfa APX 100 are necessary.
Among these, my own bias is towards the "old tech" emulsions, especially if her subjects are portraits. Not to denigrate T-Max or Delta, they are indeed sharper and finer-grained, but I find their look is just less flattering overall. And FYI, my own informal testing (with Minox negatives in D-76) has found that APX 100 is the finest-grained and sharpest of the classic 100-speed films... at least in MY darkroom.
As to where to get it developed and printed, keep in mind that you get what you pay for. A pro lab that does conventional b/w and poster size prints on a regular basis is your best choice... but it'll cost ya.