If you have any party stores, check there. You might luck out and find some deep discount Halloween stuff.
Check out fabric stores also. You can find Disney fabric that you can buy by the yard for pictures. You can find ribbons that might match different characters in feeling or design. I bought some red with white polka dot ribbon recently that looked perfect as Minnie Mouse ribbon. But the big fabric stores will also have fancy ribbons or fabric that will remind you of Cinderella's ribbons, etc.
Buy a pack of construction paper from a Dollar Store or Walmart! Cut out circles to make Mickey Mouse heads and spell out Happy Birthday using a head for each letter and make a banner! Use other colors that appear on Mickey (black, red, yellow) to cut strips and make a chain to hang up on the ceiling! Use the other colors to make little cutouts of your favorite characters. We did this for my birthday last year and none of the decor was professional looking but it was adorable and everyone loved it and asked what boutique we bought them from. You can also make a giant Mickey Head and then get a silver sharpie and have guests sign the giant head, so you can frame it and keep it up in your room.
Also hit up Goodwill and look in the kids section to see if they have big bags of Disney character figures for sale. Ours usually does. We bought a $5 bag and it had about 20 different pvc Disney characters. We arranged some around the kitchen and living area, and super sterilized a few to put on my cake.