Just tell yourself two things:
1. Cosplayers came in knowing full well they'll be photographed
2. Cosplayers want to look good in the photographs, too
Keep those two in mind, it'll be easier to ask for a photo.

Also, if you ask, it's easier to ask them to strike different poses, too. Of course, obvious etiquette applies (don't ask if they're in a hurry, eating, etc).
For me, the hardest part is if the cosplayer is in a group, and they're all from different series. I prefer to shoot them solo (especially considering the focal length I use), and they tend to prefer group shots during that case. Boo.
I need learn how to make them pose nicely. That's what's stopping me from asking more. :sticktong Also kinda hard to ask for email to send pics. Feels creepy lol. I just put it out there and hope they notice and ask for it. Haha

Maybe when I get my own lightstand (probably never).