I've had good success from filterfind.net. Tim knows his stuff; he found me a series adapter for my Zeiss Ikon Contessa which has an EXTERNALLY threaded lens! Why, Oh Why? For BW film I'd agree with prior posts, Yellow (be advised there are at least two different "yellow" filters!), red, orange, green, and polarizing although orienting a polarizing filter is difficult unless with a SLR. Go for a circular polarizer, it will work on any camera. For all of these be aware of the exposure increase needed for other than a skylight or UV filter. Of course TTL metering takes care of the "filter factors."
Don't forget proper lens shades! They can make anywhere from a subtle to amazing difference in your pictures.