This answer is a quick way of saying that the tickets are difficult to find/buy.
When it is 'hard to come by' something it means that it is hard to find.
これは、そのチケットがなかなか入手できないことを伝える簡潔な言い方です。
'hard to come by' は「入手するのが難しい」という意味です。
In American English we use the phrase 'sold out' to mean that a certain item or product is no longer available because so many people have purchased it. 'Sold out' can also refer to a person who compromises their values or ideals for money or greed. We might say that such a person has 'sold out'. Another phrase is 'buy out' which means to 'buy everything all at once', e.g.: "Large Marge was rather rotund, and would buy out the whole grocery store on Sundays.'
アメリカ英語では、特定の商品が売り切れることを 'sold out' というフレーズで表します。
'sold out' は他に、お金や欲のために自らの価値観・理想を曲げる人も表します。
'someone has sold out'（〈人〉が主義を売った）などという言い方をします。
他に、'buy out' というフレーズもあります。これは「一度に全てを買い尽くす」という意味です。
例えば：
'Large Marge was rather rotund, and would buy out the whole grocery store on Sundays.'
（ラージマージは丸々と太っていて、日曜日にはスーパーの商品を買い占めていました）
It's really hard/difficult to get tickets for this concert
If something is not easy to do then you would say it is 'hard' or 'difficult' to do by adding really it means it is alot more so very hard/diffcult to do
When going to a concert you have to buy 'tickets' to be able to get in
When you say that something is not easy you are effectively saying that, the particular thing that you are attempting to do is difficult.
You are also able to use any other word in place of "hard" such as difficult, arduous, stressful
for example:
The concert tickets are difficult to get.
"not easy"（簡単でない）は、つまり「難しい」という意味です。
"hard" の代わりに使える言葉は他にもあります、例えば "difficult" "arduous" "stressful" など。
例：
The concert tickets are difficult to get.（このコンサートのチケットはなかなか入手できない）
"The concert tickets are really hard to get." is a really simple and straight forward way of explaining the difficulty of either finding or buying the tickets due to a few number of them available.
If you would like to make a more fun exclamation, you can use the metaphor 'gold dust', which means that they are rare by saying "The tickets to the concert are like gold dust!".
To talk about the action of trying to get the tickets rather than talking about the tickets you can say "trying to get tickets is tough."
"The concert tickets are really hard to get."（そのコンサートのチケットはなかなか手に入らない）は非常にシンプルでストレートな言い方です。「チケットの数が限られているためなかなか手に入らない」と伝えています。
楽しい言い方もできます。「貴重なもの」という意味のメタファー 'gold dust' を使って、"The tickets to the concert are like gold dust!"（そのコンサートのチケットは、砂金みたいなもの［すごくレア］）。
「チケット」ではなく「チケットを手に入れる行為」について言って、"Trying to get tickets is tough."（チケットを手に入れるのは大変）と表すこともできます。
アメリカ合衆国 
イギリス 
