My son is finally ready for school at 18 months! Where can I buy a backpack for a toddler and any recommendations on brands you liked?

No particular brand. U can see which character he likes and get for him what he prefers. In this way, you are also encouraging him to go school. Had tried in my kids and it works. Initially get what is affordable but he declines and hate going to school as he doesn't like the bag he had.