Say that there was an error reading the card.
If the card is not formatted properly (FAT16) you will get the "error reading" message. If you get the "not found" message then probably the adapter or the JB is bad, check for damage to the connectors.
Yes, you can buy the adapter directly from Mattel, look for the topic "order juicebox mmc adapter directly from Mattel" in the I-Appliance BBS message list (currently 7 or 8 messages below this one). They have the adapter, card reader, and 32M SD cards available.
If there is a K-Mart or Sears near your location you may want to check the clearance rack in the toy department. I was in a "Sears Essentials", formerly a K-mart store, two weeks ago that had MP3 kits ($10), Juiceware carts ($6-$8), and even one Juicebox in a rather worn package (still marked $59.95).
As others have pointed out - KB Toys (JB, MP3 kit, 3 carts, etc. all in a box for $15) and Toys-R-Us are also worth checking.
If all else fails there is always ebay.
Hope this helps.