I also use a Weller, but mine is a wand, not a gun. If you have a wand, you can use tips from any other wand type.
Does yours look like this?http://www.highlandwoodworking.com/index.asp?PageAction=VIEWPROD&ProdID=6979
If it does, you can purchase the tips in most woodworking stores or craft stores.
If you have the gun type, you can still buy tips, but only from a specialty store that sells Weller. You can also buy them online.
That is what mine is but I only got the one with 4 or 5 tips. Thanks, I had looked online and couldn't find any and was starting to wonder if they even sold extra tips for those types. LOL I will see what stores I can find around here.