You might want to try small flyers on colored paper (to draw potential students' eye) on the gun counters of some of the "big box" sporting goods chains, like Bass Pro Shops, Cabelas, Scheels, Sportsmans Warehouse, etc. Of course, it'll largely depend on how receptive the local manager is to having such items in their store.
IF you offer women-only classes, make up a separate flier, print it on different colored paper, make the women-only notation a large bolded font near the top instead of buried at the bottom. There are many days lately I see more women at the handgun counter than men. Many I've spoken with say they're reticent to take a class with a "bunch of men who've been shooting all their lives" (their perception, right or wrong) or would be less likely to ask questions because they didn't want the "more experienced" (again, their perception) men to roll their eyes or scoff.
Iowa became a "shall issue" state as of 1/1/11, so lots of first-time gun handgun purchasers are going to these stores, and at least for us the managers have been pretty good about putting out the fliers at no cost. Some don't want fliers littering their gun counters (which we fully understand) but will allow you to post fliers or small informational signs on the bulletin boards at the front of the store. Just be aware that some of the bigger stores have their own teams (or a formal arrangement with a transient team that goes state to state) that teach carry permit classes, so they're not too keen on putting up a competitor's information. Talking to the store manager can lead to good things.
We've tried newspaper ads with varying degrees of success. We're moving away from advertising upcoming classes with an expensive three-day run (with no guaranteed position) to a weekly "Professional Services" type ad grouped with others into a larger half-page type spread that's in the same position every week. We had great success with that type of ad for our (main) computer repair business, so we're crossing fingers it'll work for the gun classes too.
As far as my own training goes, I found a great instructor through my FFL dealer. Word-of-mouth is usually some of the best (and cheapest) advertising out there.