Before you buy any more fish, buy a liquid test kit and post the parameters so we can help you with your gasping fish issue and you don't lose anymore fish.
Now on to your question, if you are always looking for the cheapest price then always search around as different fish will be cheaper at different places, but if you are looking to start a relationship with a LFS (Local Fish Store) then spend a few extra sheckles and buy there. My LFS is always the cheapest around and knows what I have so when I ask about something, he has said to me "you can buy that if you have another tank to put him in" and that has saved a few headaches. Also his fish are acclimated to my water as we get from the same source.