Does the flea market sell sports card supplies, i.e. card holders, cases, etc, ?

I'm sure they do. Someone said it takes at least 4 hours to walk through the whole place and browse.

All of the above. It's a large flea market

It is a huge place so it is possible that they have them there.

Yes you just have to find the shop that sells it

Probably. The place is huge and has about 1000 shops.