I found a lot of places that might have the metal tips, but haven't ordered any nor shopped for them.
http://www.clotilde.com/list.html?cat_id=18 has something.
SoftExpressions.com is another one
Mayco sells an 18 gauge applicator which is too big. I didn't find any other sizes there.
Annie's Crafts has Glue Caps and Needle Tips.
Michael's might have the metal tips by the stained glass/glass painting section.
Some JoAnn's stores have the tips with paint bottles and brushes. Apple Barrel acrylic paint has a tip.
Hobby Lobby might have them in the craft department.