Why is there both 另外 and 别 in the sentence 你还是另外找别人吧, and why does 另外 come before 找? The following example sentence is from the book 标准教程 HSK 4上 (p.32):




  最近小雨太忙了，没时间帮你，你还是[另外/另]找别人吧。


It means something to the effect of Recently Little Yu is too busy and has no time to help you, you had better find someone else.  It's an example of where either 另外 and 另 can be used.

However, I don't understand why 另外 (or 另) comes before 找 (are we searching again for help?), and I don't understand why we need both 另外 (or 另) and 别, which both mean another.

Question: Why is there both 另外 and 别 in this sentence, and why does 另外 come before 找?
It's a very hamfisted example of what the book is trying to illustrate -- that 另外 and 另 can both be used to modify a verb, but that 另 is restricted to modify only monosyllabic verbs. 

To answer the first part of your question: 另外/另 before 找 because it is specifically modifying the verb 找 using 另外/另 (note that you cannot use 另 for non-monosyllabic words according to the book).

The second part of your question involves the meaning of 别, which here, I believe you should take into account that 别 attaches to 人， so that you have 别人 as one word meaning "another person" (or more specifically "a person that is not already been mentioned")

In that case, 另外 is modifying 找, emphasizing that it is an additional and different task to be done, whereas 别人 mentions another person. I feel like it means something along the lines of "you should rather (do the additional task of) finding another person"