Well, one of my first recommendations would be the TRULY old-fashioned way: going to a bookstore and finding books that are like yours (similar subject or category), and writing down who published them.
Armed with that info, I'd Google the publishers and look for their guidelines. The reason that's probably the best method (though more time-consuming than simply using a directory) is that it gives you a much better idea of which publishers are actually capable of getting books onto bookstore shelves. It can also show you which publishers pay for co-op space (the books that are on endcaps and on tables in front of the store), and which ones tend to sell well-- look for books that are face-out and stocked in multiple copies.
Directories don't tell you that... and I have yet to come across a perfect one that doesn't have some vanity presses or useless publishers mixed in (publishers that don't have any distribution). If you were considering FirstWriter, don't-- they don't vet their listings at all.
I also found small publishers that I ended up going with twice in the (printed) Writer's Market, by using their index to search by genre. I then read the guidelines for all the publishers they listed that published my genre, visited the publishers' websites, etc. One of those was for the educational market, so bookstore sales wasn't the goal.
I like PublishersMarketplace, too, though many publishers don't report there, so it's not a comprehensive look at who's buying what. If you're just using it to search out who published what books, you could do the same with Amazon searches.
Thanks, Jenna! I had already done an Amazon search. Unfortunately my client insists on starting with the largest publishers first. I tried to get him to do the bookstore route earlier, but you've given me more ammunition to pitch that ploy again. (I'll quote you.)