Are you signed up online? If so, you can go into fundraising tools and print off a Great Strides Sponsor form and submit it at the walk. Otherwise when you show up at the walk, just stop at the registration table and they'll help you out.
I have a friend who doesn't like to carry around the cash, so she writes out a check and then fills out the sponsor form with the names and addresses and amounts of the people who've donated. And last year my grandma wrote out a check to me, so I just donated to my son online in her name.