First off, do you have a form letter which states school information, contact information, info on the fundraiser, who will benefit, your school's tax exempt #, etc.? You need to leave these behind for managers to review, they will call you back. This will benefit you greatly, for local stores, restaurants, etc.
Do you have some parents lined up just for Corporate Sponsors/Donations? It helps to have a team out there, each should have a list of local and national companies they are willing to contact. Do you have any parents who are really friendly and maybe in sales for a living? It is vitally important to start far in advance especially with National companies, they need lead time.
I just finished a large fundraiser and found that walking into our establishments and asking face to face gets you a "Yes" most everytime! I also found some companies like to work via e-mail, try this as well. Ask for coupons (win win for everyone), product, parties and services. If you do get a "No", do not be discouraged, keep trying for the next event. Walk out with a smile on your face and understand that companies can only donate certain times of the year, for certain functions, etc. Maybe they can't donate right now, but they will hopefully remember your friendliness and smile for the next time.
List of my local nationwide companies who gave to our fundraiser:
Subway, Walgreens, CVS Pharmacy, Sports Authority, Best Buy, McDonalds, Harley-Davidson, The Jump-Zone, Papa Johns, Superbowl, Oberweis Dairy,
Other places to ask for donations: local grocery store, local Hospitals, banks, ortho, dentist & doctors, landscaping companies, home party buinesses, local library, car dealerships (Just bought a BMW not long ago, went back and asked our salesman for a donation from the BMW store) any place you think would benefit by donating to you, be open minded and get your name out there.
Lastly, do you have a school business directly, which shows either parents who own companies or friends of the school who advertise in your directory? This really helps...lets you know who does what in your school and who better to ask than them. Ask around for anyone with connections...my brother lives next door to Ozzie, Manager of the Chicago White Soxs, of course I ask him all the time to get stuff for us!
You never know who has connections, just don't be afraid to ask.
I hope I have given you some ideas...most of all, have fun with it, it will show in your enthusiasm and people love to see a smiling person walk into their establishment. Good luck!