Here are a bunch of resources you can use for Probabilistic Method after going through the theory on your own/lectures [MIT OPENCOURSEWARE LECTURES are recommended by me]
https://www.math.cmu.edu/~ploh/docs/math/mop2011/prob-method.pdf A nice po shen loh handout. Try the question here for practice.
(in general, there are several nice handouts at https://www.math.cmu.edu/~ploh/olympiad.shtml) (harder version: https://www.math.cmu.edu/~ploh/docs/math/mop2009/prob-comb.pdf)
Chapter 9 of Pranav sriram. You can get this with a quick google search or in post 49 in this aops thread: https://artofproblemsolving.com/community/c6h601134/
Check the counting in two way handout at https://yufeizhao.com/olympiad/
Evan chen’s handout: https://web.evanchen.cc/handouts/ProbabilisticMethod/ProbabilisticMethod.pdf
Also the Global Chapter in OTIS Excerpts, which you can find on evan chen's website here.
I compiled these resources from various posts about Probablistic Method, the major one being OMC, so credits to them.