Since most "Public Radio" stations purchase their programming and develop their schedule to suit their own needs, the suggestions above about a) donating money at pledge time during the folk program(s) you like and b) making it known that you are supporting the radio station because of the folk music programming are probably your two best bets. If you get hold of a local music org and get THEM involved doing the same thing, you will have as good a chance as any in maintaining, if not increasing, the level of folk music you get.

Where I am, I get four public radio stations. Each has a different format and different audiences - all of them get support from me based on what they provide that I am interested in.

Good luck - Pete