A very naïve but logical question:
Why are these missing in the Hiragana chart?
If there are, where can I find them? How to write them?
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They're not "missing", these hiragana characters aren't needed as they don’t exist in modern Japanese language. The language doesn't have these sounds so they did not need to be represented. You cannot write them in hiragana.
Some additional conventions exist to write foreign sounds in katakana but there are still limitations as both systems were originally designed for writing Japanese words. Until recently, these characters were only used to represent sounds that exist in Japanese that can be spoken by a Japanese reader. For example "wu" in chinese names is pronounced as "bu" in Japanese, "gim" in Korean names is pronounced as "kim" in Japanese, "va" in European names is pronounced as "ba" in Japanese. Furthermore "fu" and "hu" or "ra" or "la" are not distinguished in Japanese loanwords from other languages.
There are several exceptions for historical reasons. "wi" (ゐ) and "we" (ゑ) are used very rarely for names but are now pronounced the same as "i" (い) and "e" (え) respectively. There is no longer any need to write them differently as the sounds in modern Japanese are the same. Some brands still use it as their original historical name such as "Yebisu" (ゑびす/ヱビス) beer.
Another source of confusion is inconsistent romanisation. The "ye" sound for example, doesn’t exist in modern Japanese but ゑ was written as "we" or "ye" since romanisation was not standardised at the time. This is also why 円 is written as "Yen" in English, it was historically read as "wen" (ゑん) and is now read as "en" えん. So despite non-standard romanisation suggesting it, the sound "ye" doesn't have a hiragana character as it is not spoken in Japanese.
These syllables simply don't appear in Modern Japanese, so there are no standard symbols to represent them. This also goes for the "we" and "wi". Their pronunciation has shifted from "we" to "e" and "wi" to "i", so in native Modern Japanese words, you can find them written as pronounced.
Even if these syllables appear sometimes in loanwords, the katakana used to represent them isn't the traditional "we" and "wi" katakana, rather the "U" katakana plus a small vowel symbol, like in ウェスト [WeSUTO] (waist)
As others have said, they're not used in modern Japanese and the characters aren't taught in Japanese Elementary schools. However, saying they don't exist isn't technically correct; they do exist in an old textbook on Wikipedia. You can find them here: ヤ行イ
Links to the specific kana charts for reference:
Edits: I misread the question originally and missed the specific context of "yi", "ye, "wu". I don't know a modern specific situation where those have been used.