Do not use the warmed up water other than if you used an aquarium heater. Warmer water not only might have chemical in there if you used it from the tap, it have less oxygen in it.
Your betta fish will be fine for a few days if your room temperature is not below 70F. Get a high quality heater is very important. I haven't had one broken on me yet.
In most cases, local pet stores only sell the least fancy betta fish. It is why they are going for like $2~3 each.
Those fancy ones like half moon can be sold for $30 or more on ebay or other places.
