You will get longer flight times if you go with a higher mah pack. But you need to make sure it will fit in the space you have. You probably need to get one identical to what you had and then experiment with other packs to see what works the best. Hobbycity.com is the best place to buy Lipos and chargers. I have a $20 Turnigy Accucell lthat works great, and they have 2 cell lipo charger/balancers for less than $10. Just make sure double sure you check the dimensions so you don't get a battery that won't fit. Also pay attention to the C rating and Burst C rating.

If the pack you had is a 30C and you buy a 20C, you probably wont be happy with the performance. With out going into a drawn out lipo lesson. the mah rating determines how long you will be able to fly, the C and Burst rating affects how quickly the pack can deliver its power. This can affect top speed, acceleration, or the amount of power that can briefly be supplied for lift, torque, etc. You probably don't want to fly for twice the amount of time with only half the power, or fly for half the time with twice the power, so this becomes important.

You said it was a 7.4 volt pack, which means it is 2 cells in series, and I'm guessing you probably had a 1000mah pack with a 20C burst rating and 15C contiunous.
To order one like I just mentioned, it would be described as a 2C2S 1000mah, 15C. 20C burst. The "2C" refers to two cells, the '2S" means the two cells are wired in series (If it said 2P, that would mean two cells in paralell and would only be 3.2 volts). The burst rating is sometimes listed with the dimensions and some times is included at the end of the main description.