Sorry, but at the current time I do not have the ability to post photos. I don't even own a camera, and I am not computer-literate enough to complete the process if I did.
Different methods may be employed to make your fittings, but the two most commonly employed are casting and forging.
Given the nature of your question, let's talk about forged fittings.
Since you mentioned brass, the first thing to be discussed is your choice of material. I do not know your level of experience, but your abilities will definitely have an impact on the material that will be best for you.
The habaki will require hard soldering (technically, this is called brazing).
The fuchi is nearly always a two-piece item that requires soldering.
The kashira requires no soldering, but the nature of the forging process frequently requires annealing for all but the most experienced metalsmiths.
The shitodome requires soldering.
The easiest to make is the seppa, as it requires no forging or soldering. It is simply cut, filed, and sanded to desired shape. The most critical aspect of the seppa is that it be of proper thickness.
Both annealing and soldering of brass is considerably more involved than doing the same to copper. Improper application of heat can ruin an expensive piece of material.
Many people new to this tend to observe cost comparisons of brass to other materials (such as copper). But this is considered false economy- yes, brass can usually be obtained easier (or cheaper) than copper, but the amount of brass you waste during the learning process (and the extra time involved working with it) makes copper the clear winner in the economics race. The exception to this would be those processes that do not require soldering or annealing (such as castings).
So if you're new to this, I highly recommend using copper instead of brass, as copper is more forgiving and easier to work.
Back to your question- the tool to which you are probably referring is often called a
stake. It is also called a
mandrel. These terms are used interchangeably; I do not prefer to get involved in a discussion as to which is the proper term, but for purposes of this discussion they mean the same thing.
Essentially, a stake/mandrel is nothing more than a tiny anvil of your desired shape. You shape the metal around the stake to obtain your desired shape.
The shape and size of the tsuka will determine shape and size of your fittings. Since Japanese fittings can vary widely in both shape and size, quite often the metalsmith will have several different stakes. As the fittings are nearly always of an ovoid shape of various size, the stakes are usually custom-made by the smith himself for each application. To the best of my knowledge, these types of stakes/mandrels are not something you buy; they are custom-made.
As I mentioned, I do not have the ability to post photos, or I would show you some of mine. But I will search the internet later when I get time if you still need photos.
Perhaps someone reading this has some stakes and can provide a pic.
Bubba-san is a member here and may be able to help, too.
In the meantime, an excellent source of information on construction of fittings is a DVD set by Walter Sorrells.
For anyone new to making fittings, this is invaluable. It's packed with information. Especially useful to you will be the showing of the various tools involved,
and how to make those tools.
http://waltersorrellsblades.com/videos/japanese-sword-fittings-video/
Good Luck,
Rob