Don't call it a bong when you go in there. Water pipe is the correct term, I believe. I never really call anything in head shops by names, I just point at the ones I want to look at.

Also, to make sure your slide is air tight, put it in and hold your hand on the top of the bowl and suck in. You shouldn't be able to suck in essentially. That means it's air tight.

Good luck and post a pic when you get it!