Sorry to say, but this could be difficult. There was a piece on GMTV this week explaining how the postal system gets hopelessly overloaded and asking that no more shoeboxes be sent (one big consignment had just gone - didn't get name of org, but perhaps that's why you're getting a 'no more until next year message). The spokesman said that they were immensely grateful that people remembered the boys and girls, and suggested that instead of sending shoeboxes, they instead made a donation to the SSAFA Welfare Fund: units in theatre can access that fund for all sorts of things (example he gave was projection equipment) to make life a bit better.
Another possibility would be to get in touch with your local branch of a Service charity which does welfare casework (such as SSAFA, perhaps RBL) and see if they are collecting for those they have assisted during the year. I don't know if every branch does this, but those that do find that for dome of their old and isolated it may be the only present they receive.
Good luck with finding something you wish to support.