While fancy guppies are generally bigger than feeder guppies, and prettier, they're also a lot more expensive. You should be able to buy feeder guppies for 20 cents or so. And the nice thing about guppies is they are air breathers, so if you stick a male and two females in a bowl of dechlorinated water and keep them fed you'll get about 20 babies from each female every month. It takes a while for the fry to get big enough to feed to your snakes, but if you have the room I feel it's worth it. Just be sure to crush the food very fine when there are babies and they should do just fine.