The Komusô-Shakuhachi is not a musical instrument per se but rather a tool for meditation. That is important to keep in mind to avoid misunderstandings. More traditional names for the instrument are "vessel for the Buddhist teaching (dharma)" (hōki 法器) or "dharma-bamboo" (hotchiku 法竹). Every aspect of the Shakuhachi relates to its playing-purpose: Its tuning it is not strictly adjusted to the pitch of the Western musical scale and this kind of Shakuhachi is only played alone - or sometimes unisono during ceremonies. The length of the Shakuhachi is not standardised but only depends on the natural growth of the bamboo used. In contrast to that, modern Shakuhachi are thoroughly tuned and their length is in most cases adjusted to one shaku and eight sun (about 54.5 cm).
Jinashi 地無し and Jiari 地有り Shakuhachi
Ji is a mixture of the natural Japanese lacquer urushi 漆 and tonoko との粉 that is a kind of grindig powder. Ji is used within the bore of modern Shakuhachi to adjust it to a strictly defined diameter and thus allows exact tuning. Such a Shakuhachi is called jiari "with ji". The inside will look smooth and even like a mirror.
The Komusō-Shakuhachi on the other hand is not tuned to an exact musical pitch and therefore ji is not necessary. Those Shakuhachi are called jinashi "without ji". In the inside of such a Shakuhachi you will in many cases - but not always - recognise where the nods of the bamboo originally were and see the natural structure of the bamboo. Jinashi-Shakuhachi also have in their inside one or several thin layers of urushi (but nothing else) to prevent the bamboo from moulding and from insects. Jinashi therefore does not mean that the inside of such a Shakuhachi is left raw without any treatment. Sometimes Shakuhachi without any covering of the inside, showing the raw bamboo surface are offered as jinashi and such a flute can also be played. But it will mould easily and - if you are living in a warmer climate - it will be damaged by woodworms sooner or later.
The two types of Shakuhachi, jiari and jinashi, differ immensely in tone and playing technique. A jiari has a clear and sometimes loud sound, and for playing it, you would have to develop a stable embouchure. Many of those instruments have the standard length of 1.8 shaku and a similar diameter at the mouthpiece, utaguchi 歌口. If you can play one, you can probably play many of them, as your mouth-position does not change much from one flute to the other.
The sound of a jinashi is quieter and softer. For playing jinashi you need to keep your lips soft to adjust to every new Shakuhachi you play, as they all have different dimensions. They come in many different lengths and what length you play depends on the tradition of your teacher and, of course, on your own taste and playing abilities.
How to get one
"Our society knows the price of everything
and the value of nothing!" (Horst Stern)
Modern Shakuhachi are sold in stores and you can find an incredible selection online in Japan and abroad. To say that jinashi-shakuhachi or Komusō-Shakuhachi are not readily available for sale would be ridiculous, as many professional makers offer those as well. But it had not always been that way.
The local playing styles of the Komusō traditions are quite distinct and one needs a certain kind of Shakuhachi to play them. Finding a teacher and asking him or her what Shakuhachi you need is - in my opinion - the best way to proceed (that is also true if you are interested in the modern Shakuhachi). Many jinashi-shakuhachi players are making Shakuhachi themselves and maybe your teacher will just give you one of those he has made. That will then match perfectly the requirements of his playing style.
I can only report my personal experience here, but I have never spend money to buy a Shakuhachi. Over the years I have received quite a number of Shakuhachi from my teachers and I regard myself as their caretaker to prevent them from moulding, cracking or simply from getting strange by lying around unplayed. Shakuhachi are generally longer around than we are, and finding someone to take care of your Shakuhachi before it is too late is important.