The best solution I have found is at Bumpyname.com
Not exactly a label,they are similar to the rubber bracelets - like the Lance Armstrong yellow live strong. Your childs name is "imprinted" on them. There are A LOT of standard names but, you can custom order any name and spelling. They also come in several covers. It has been several months since I ordered them, but, I think the custom orders are slightly more expensive. They are called bumpy because you can also have them done in Braille.
I have been using these for bottles at daycare but, they also work perfectly on the sippy cups I have just started to introduce my 7 month old daughter to. The daycare ladies seem to like them as well. I hope this helps! C.