Sponsors can provide venues, refreshments, products, free services, or money to help your fundraiser. However, to get these sponsors you will have to convince them that your fundraiser will be successful and your cause is worthy of their effort. Think about local businesses that share your values and might be willing to support your fundraiser. Then, contact them by phone or in person and make your case.
You may also be able to take advantage of contacts within your team to gain the support of business. Hold a brainstorming meeting with them to figure out potential sponsors.
Consider having sponsor levels (gold, silver, etc.) that reflect the amount of involvement sponsors have. They may be willing to pay more to have better placements of their logo around the event.
For example, your charitable 5k could try contracting local cancer-focused charities, hospitals, and athletics stores around town. These organizations share a common interest with your cause and the people who will be attending your event.