I just got the 8 pack in of those, comes with 4 adult and 4 kids. They seem pretty flexible. Back in 2012 I order 3 tubes of the LG cinema 6 packs plus the 4 that came with the TV and they are now all either lost or destroyed save for the clip ons so it was time for some replacements. The LG ones seem to break real easy.
ebay or just take family to see a 3d movie and keep the glasses
I'm not sure if they're all compatible, but I have bought extra Sony ones and they are much brighter than the theater ones on my home 3D TV.
I tried taking some home because the movies has smaller kid sized frames, but the picture is much darker.
because they are passive 3d glasses, you can use any kind, they all will work.
passive 3d glasses use a polarizing filter so just buy some cheap one, but make sure they sit comfortable on your head, especially for longer movies