I included other sentences I turned into indirect speech. Where can I find exercises on the use of indirect speech so that I don't have to invent all the sentences myself? Thank you.

1) Why don’t we go for a walk? She suggested going for a walk.
2)I gave you my homework last week. She insisted giving me her homework the previous week
3)If you do that again, I’ll leave. She threatened that she would leave if I did that again.
4)“Please, give me another chance. She begged me to give her another chance.
5)“Who bought the Picasso painting?” She wanted to know/wondered who had bought the Picasso painting.
6)Do you work in the central branch or in the provinces? She wanted to know if I worked in the central branch or in the provinces.
7)It wasn’t me who stole the money. She denied that she had stolen the money/stealing the money.
8) She congratulated on passing my exam. (is "congratulate" always followed by "on"?)
9) She insisted on paying the bill.

http://grammar.ccc.commnet.edu/grammar/marks/quotation.htm#silent_speech

Read and study all about indirect speech and the uses/placement of quotation marks.

http://grammar.ccc.commnet.edu/grammar/marks/question.htm#indirect
These are mostly about indirect questions.

http://www.englishclub.com/search-results.htm?cx=partner-pub-3505543279168530%3A39pxe6-t2e8&cof=FORID%3A10&ie=UTF-8&q=indirect+speech&sa=Search+EnglishClub.com&siteurl=www.englishclub.com%2F&siteurl=www.englishclub.com%2F
There are several links here that will give you ideas for sentences/questions.

1) OK

2) I gave you my homework last week. She insisted she had given me her homework the previous week.

3) OK

4) "Please, give me another chance." She begged me to give her another chance.

5) "Who bought the Picasso painting?" She wanted to know/wondered who had bought the Picasso painting.

6) OK

7) "It wasn’t me who stole the money." She denied that she had stolen the money/stealing the money.

8) She congratulated me on passing my exam.

(is "congratulate" always followed by "on"?) <~~Yes, usually.

9) She insisted on paying the bill.

To find exercises on the use of indirect speech, you can try the following resources:

1) Online grammar websites: There are several grammar websites that offer exercises on various grammar topics, including indirect speech. Examples include EnglishGrammar.org, Grammarly, and Perfect English Grammar. These websites usually provide explanations and examples along with interactive exercises for practice.

2) English language textbooks: Many English language textbooks, especially those focused on grammar, include sections or units on reported speech. These sections often contain explanations, examples, and practice exercises for learners to reinforce their understanding.

3) English language learning apps: There are several language learning apps available that offer comprehensive grammar lessons and interactive exercises. Some popular apps include Duolingo, Babbel, and Rosetta Stone. These apps usually have dedicated sections for reported speech or indirect speech where you can learn and practice through quizzes, fill-in-the-blank exercises, and more.

4) ESL/EFL teacher's resources: If you are studying English in a formal educational setting, your teacher may provide you with additional exercises on indirect speech. Teachers often create their own handouts, worksheets, or activities tailored to their students' needs. Make sure to ask your teacher if they have any additional resources for practicing reported speech.

Regarding your eighth question, yes, the verb "congratulate" is typically followed by the preposition "on." So, the correct sentence would be "She congratulated me on passing my exam." This is a common collocation in English.

For your ninth question, the sentence "She insisted on paying the bill" is already in indirect speech. The original direct speech version would be "I insist on paying the bill." In indirect speech, the verb "insist" is often followed by the preposition "on" to indicate the insistence on a particular action.

I hope these suggestions help you find suitable exercises to practice indirect speech!