I agree with TA, can't you just get store credit or something until your tank finishes cycling? And didn't you say the majority of the bettas there looked ill.. doesn't sound to me like a place to trust, the last thing I'd do is add another. Chances are, if the betta had a disease before you bought it, it's been introduced into your tank so keep an eye out for that.
Either that, or your ammonia's actually at high levels and this coupled with the stress of adding the betta to the tank may have been what killed it. I'd definately invest in an API test kit, those strips, as far as I know, really don't work that well at all. Until you can get one though, like TA said, take a sample of water to your LFS and get them to test it.