I think Microsoft didn't think to have any spares made. There are no replacement tips for the pen either and you have to contact Customer Service to get them.
That glob of glue you mentioned is most likely the adhesive that was supposed to be on the tab and it rolled off. That is pathetic manufacturing and quality control.
Microsoft needs to get manufacturing here in the states and break away from China. China doesn't like them anyhow and China is stealing the technology from the computers to use in their space program and defense systems. Besides, China is about to make a modified version of Ubuntu as their only system authorized in the country and other systems will be forbidden in the near future. That is where most of the hacking and bootlegging is coming from anyway.