Some schools require a certain number of shadowing hours. Call each school you plan to apply to, or check their website to find out. I would strongly suggest shadowing anyway; it'll give you more information on the profession, and ultimately help you in both your personal statement, and your interview.
If you have a pre-optometry club at your school, ask around, and see who other members have shadowed, or are volunteering with. You can shadow your own optometrist, or simply look up local optometrists, and call them to see if they'll allow you to shadow.
Again, some schools require a letter from an optometrist, and some schools don't. So look up the information on this too.