hi, i'm not an expert, but i've been reading about and playing with these filters recently, so here are two links that may be useful to you:
the easiest place to get these filters would be cathay photo. they only sell cokin and lee ones. the lee filters are very expensive, and you may want to just start out with 1 or 2 cokins to try out.
get the cokin P system, not the smaller A system. the holder costs between $10-20, can't remember exact price. there's a wide-angle version too. the adapter rings (you need to get one that fits your lens filter thread) cost around $16, and the filters cost around $28 each. some people find the holder too troublesome, and just hand-hold the filters in front of the lens.