Whenever I make buildings in GECK or CS I always use pre-made buildings but now I want to make may own casino and I noticed that there was no pre-made interiors for them. So were the casinos made with kits? If so where can i find them?

Well, there are some pre-made casino interior parts under "cas" at static.

I don't think any of the casino interiors are 'premade'. In the object window; in the tree find World Objects> Static> dungeons. Then you will see a few options like NV_CAsino_TOPS, etc. These contain most of the kits used to make the casino's. If you look around the area under Dungeons you should find all of the kits used to make any casino.