At Michael's, I believe I found them hanging on a hook near the scrapbooking/paints area. (Someone else mentioned where the stained glass and glass paints are in Michael's.) Very easy to overlook because they are so small.
I found the plastic bottle with metal tip at Amazon:
http://www.amazon.com/Ultrafine-Appl.../dp/B000WWM6R6
I've also come across the tips themselves on paper crafting sites -- I'm thinking quilling or scherenschnitte. If I find one of those, I'll post the link.