Seconded, fancy guppies are notoriously inbred and weak. Years ago guppies were considered very hardy - however times move on, and inbreeding has led to weakness.
Either way - I'm going to be getting more, but I'll be setting up a small tank specifically for them. Thinking about dwarf gourami's instead for the community tank - another sickly fish, but big and ugly enough to look after itself and not take any rubbish from my (occasionally aggressive) Rams.
You
can't get anything other than fancy guppies. The term "fancy" in the pet trade is just another term for domesticated. Guppys are hardy fish, this recent idea that breeding has made them weak is misinformation, domestication has made them hardier to aquarium conditions, not weaker. Inbreeding is an entirely different matter. If anything, their elevated exposure to novice fishkeepers is more likely to account for their high attrition rate. A decent, reputable LFS with good healthy stock is usually enough to ensure you are buying a good strain.
If you're having trouble keeping your guppies then I'd think twice about moving on to more advanced fish like gouramis.