Folks here can give you a few suggestions if they know more details. Here's a good start:
What are you planning to cast?
Are you planning on making multiple casts of the object or do you need just one?
What materials was the original (the buck) made out of? (Some materials will stop certain materials from curing)
If it's an uncomplicated item (one without many undercuts, etc) you probably can get away with a plaster mold as being the cheapest option. If it's going to be more complicated, then you're looking at a flexible mold (out of a tin-based or platinum-based silicone). There are also some rubber molds but I'm not as familiar with them.
The more information you can provide the better. And the Smooth-on website is a goldmine for instructional videos. I'd start there.
It depends on what you are making, but one general rule I was taught and have learned holds quite true is never cast like into like. So if you want a hard cast (say fiberglass, plaster, hydrostone, etc.) then use a soft mold. Conversely, if you want to cast a soft material (latex, silicone, etc.) use a hard mold. Now this is a general rule to get you by in most situations, but I've found it holds for most cases and you really shouldn't ignore this rule unless you get more experience and know exactly what you are doing.
I can tell you first hand that trying to separate fiberglass from a plaster mold is not easy and you are almost guaranteed to break the mold (which might not be a bad thing if you only want one, but generally it's never good to break your molds).