How to get a list of kinases that phosphorylated a particular protein?
I use PhosphoSitePlus too. I use it in combination with ELM as it finds linear motifs and generally gives a good generic candidate.

The literature if not. Checking all close homologues sometimes pays off.

Checking protein protein iteraction databases such as StringDB or IntAct may help too, but that gets very iffy —XL-MS (a smoking gun) is rare and does not pick up low concentration enzymes like kinases.

(A note: this question ought to have been asked in bioinformatics SO)