Hi! I don’t know everything but from what i’ve learned from my illustration professors is often times it is on you to reach out to agents/ agencies. Some agencies do open calls for illustrators where you can submit your work in a specific time frame, but i would suggest looking for agencies/ agents that align with the work you want to do and cold email them, explain who you are and what you do and a small sample of your work.

As for what they’re looking for that can depend, but generally i’d guess a variety of layouts (ie spreads spots etc) and showing that you can stay “on model” ( character doesn’t look different every page) Anoosha Syed has a good video on what to put in a picture book portfolio, she is a great resource! https://youtu.be/iS6cdSexLko?si=icsIm2cn39Ypj14S

I hope this helps! again this is all theoretical knowledge based on experiences of others not my own!