I would start by letting people know you are looking for an internship-colleagues, parents of friends, your teachers/professors, and family. Search message boards-both digital and in person at various departments or clubs. Also find companies that you would love to work for and search their websites for postings, even reach out to the HR department. I am not sure if they have 'internship' fairs like job fairs but that would be worth a search. Some alumnae departments at prep schools and colleges/universities post opportunities, so you might ask your parents or grandparents to look with you. Good luck! Keep sending out your information and staying in contact with people along the way because you never know when an opening will develop and you want to be the first person that comes to mind.