Many advertisements for show guppies are placed by individuals who have never competed in an International Guppy Show. These advertisers typically begin by purchasing some breeding stock, then they try to raise and sell them without the knowledge or expertise necessary to produce and maintain show quality guppies. As for the other advertisers...I can tell you which are the genuine, reputable breeders, and which are not. Raising show guppies has been a hobby of mine for over 37 years. If you are looking for color varieties that I don't have, I can refer you to a reputable breeder who does.
You may have read elsewhere that you should obtain only virgin female guppies for breeding. Please disregard this advice, as it is outdated. Typically, I send young female fish (3-6 months old) that have been pre-bred. This way, their young will be born in your tanks in about a month or less, quickly adapting to your water conditions. This is the key to getting your guppies started. Also, instead of sending older, fully-caudaled males, I recommend selecting younger males from prime stock. Younger males will adapt to your water more readily, are more active, and will chase--and mate--with your females. Males like these will be young enough that you may back-cross if you desire, although it is not necessary at this time.
I strongly recommend sterilized tanks (bare). My water conditions are hard (400ppm), and alkaline (PH of 7.6) which is ideally suitable for my show fish. All strains should be kept separately. Live, newly hatched, baby brine shrimp should be fed daily, along with regular flake foods. Never feed worms of any kind, as they may carry diseases. Do not mix your show guppies with other guppies or tropical fish until the fry are accustomed to your raising conditions, and then do so at your own risk.
These strains represent decades of hard work by various breeders, including myself, and I'm sure that you will immediately notice the difference in quality between our show guppies and fish of commercial or pet store strains, which are no match. All fish are guaranteed live delivery; you only pay for the reshipping.
Having read this, it is now up to you to improve or maintain whatever strains of guppies you are planning to acquire. Keep in mind that your results may vary depending upon the breeders selected, and your care, feeding, and water conditions. You can enjoy your guppies on many levels, ranging from keeping beautiful show guppies for your own personal satisfaction, to joining a club and showing your guppies at shows. Finally, you can sell your surplus guppies at pet stores or tropical fish wholesalers. It's a wonderful hobby, and it's your choice as to the level you take it.