The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
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Aquabid has the most variety and most affordable guppies I have seen anywhere. That's a great place to start looking. Look at the ratings for the seller.
Sometimes I browse what's for sale just to drool at the pretty guppies! You never see them like that in Petsmart and they often sell breeding trios so that you can start breeding them too!