Did you mention what state you are in?
You should see if your local library has any book sales between now and when you have the event. Often times they are willing to donate unsold books for causes so you might be able to request any unused sports books to use as raffle prizes.
You can also see if you can purchase any larger lots on ebay or craiglist and break it down into small lots of 20 or more cards and offer them as grab bags.
Another idea, see if you can find any auctioniers willing to volunteer. You can offer up an auction where people can sell unwanted cards with a percent going to the chairty and they get the rest since it's their items being sold. My son and I used to go to a sports auction in NH and had a blast. Everything from unopened boxes of cards, higher value cards, odd ball lots, unique items, signed items etc.
And of course, autographs... sometimes you can find local sports figures who are willing to donate their time signing autographs to help charities. Might not be a top star but it might be a minor league player or someone who made it a year or two into the majors.