Hello there; I have had to study combinatorics for a while and I found these books kind of useful, you can take a look at them:

— Principles And Techniques In Combinatorics (by Chuan Chong Chen, Koh Khee Meng)

— Mathematical Circles — Russian Experience (by Dmitri Fomin, Sergey Genkin, Ilia V. Itenberg)

I think these two books are good for starting with combinatorics, also there are many combinatorics questions in different national olympiads of mathematics, and there are some mathematical Tournaments containing great combinatorics problems, some to name can be:

— Harvard–MIT Mathematics Tournament

— Math Majors of America Tournament for High Schools

— Princeton University Mathematics Competition

also I have been using Iran's National Olympiad in Informatics questions, because the first two rounds of it are all about combinatorics, I don't know if they are translated into English or not, but they are useful anyways.

guess these can be enough for the start :)

This Book is helpful: A Path To Combinatorics For Undergraduates by Titu Andreescu

I have a pair of nice problems https://atcoder.jp/contests/abc171/tasks/abc171_f https://atcoder.jp/contests/abc110/tasks/abc110_d

You can try this Link

Try EnumerativeCombinatorics, it's a great book.

Some useful sets of problems (in general)
https://progvar.fun/problemsets?fbclid=IwAR2AplbbVxKLG11ktqAyy87d6CxRP9BqsW4MzxYUP9r1508DZX188253drE
(Refer to Counting sections)

Try this book Naruto, http://www.mateinfo.net/_files/downloads/tomescu.pdf It's old but gold, even diamond-ed.