Freshwater Puffer Fish Experts Needed!?

I was going to buy a dwarf puffer fish this week, but found them difficult to find in my local pet shops. I finally found 2 at a store and I'm afraid they'll get bought---so I'm going to go buy them today. So, my question is, how long can a puffer last in the plastic carrying bag they come in? I have a 120 gallon at home, but I need to buy sand today and let it filter for a few hours. What should i do? I have smoothed stone pebbles, but after much research, i have found that Puffers do better with sand? Should I put them in the tank anyways? Or buy Sand today and let it filter (If so, how long should they be kept in the bag? Should i transfer them into another temporary containter? thanks!