If you went through a formal court proceeding to get the guardianship, I would check with the court that issued the guardianship papers (in Georgia, it's the county probate court). They should be willing to provide you with those papers for a reasonable fee.
If it was not a formal guardianship done through a court, but instead your child signed papers to give you custody of your grandchild, I would check with the attorney who prepared the paperwork. Hope this helps. Enjoy your trip to Spain!
Go to the Court that issued the letters and request another set of certified letters.
No legal representation exists by virtue of this answer. It is recommended that you contact an attorney directly for a more complete response.