Unobtainable means you cannot get it, no matter how you try. Use it for things that are supposed to be gettable but are not: a medicine that no pharmacy in the country stocks, a ticket that sold out in four minutes, a permit the office has stopped issuing, information a company refuses to release. That is the flavour — a door that exists but will not open for you. It is formal, and you meet it far more in writing than in speech; in conversation people say 'you just can't get it' or 'it's impossible to find'. English also uses it about people, and there the meaning shifts in a way learners like: an unobtainable person is someone you are attracted to who is out of reach — usually the phrase is 'unattainable' for a goal and 'unobtainable' for a thing, but people mix them, and both are heard. If you want the safest rule: unobtainable for objects and supplies, unattainable for dreams, standards, and goals.

拿不到的、弄不到手的。台灣日常會說「買不到」「調不到貨」「搞不到」——重點是「本來應該有，但你就是拿不到」。若講的是目標、理想拿不到，英文比較常用 unattainable（遙不可及）。