Which brand? As for Star/Ares L85's, G&G's Version, and ICS versions they all come off. You should be able to get them separate as well
A few links:
http://www.starairsoft.com/index.php
http://www.icsbb.com/web_e/html/02_p...d1=12&kind2=17
Anything else, just hit up google...
cheers