I got mine from Peltons direct online as last year they seemed to be the only stockist, but I've seen they now sell them via amazon as well. If you google Ear Defenders then you should get a link or three.
She didn't need them at night, tbh there was so much activity and fresh air during the day she was out like a light as soon as her head touched the sleeping bag! I borrowed them, though (note to self, buy ear plugs this year)!
wrt the evening shows, we were 'fortunate' in that only 2 of the nights had headliners we wanted to see, so one night was an early one. Also we went in a group and she had her 2 cousins to play with, which made a huge difference. We set out a blanket on the edge of the arena (where we could still see the stage), opened up the winebox, gave them some glowstick bracelets and those illoom ballons (cannot recommend glowy things highly enough at festivals!) and they were so excited at being up after dark that they ran around us quite happily playing until it was all over.
We had one late night with DD on her own as the others didn't like the headline act, she was actually fine and again loved being out late at night, but a little grumpy/sleepy on the way back to the tent, wanting to be carried all the time. Now that we have a ridiculously heavy bell tent, I have invested in one of those wheeled garden carts to lug stuff from the car, and, as I saw a lot of people doing, am going to line it with her sleeping bag to take her back to the tent if we do another late one (which as Suede are headlining, is almost definite!).
I think you have to accept that your moshing days are over at that sort of thing and just make the best of being 'civilised' on the fringes. I found Latitude very laid back and child-friendly - no-one minded that there were children running about at all hours! I would imagine that Bestival is even more that way inclined...