The best overall flashcard would be the Everdrive GB. There are various versions, with a few key differences, which you can learn about on the store page:
https://krikzz.com/store/home/46-everdrive-gb.html But basically the X7 has all features, the X5 has no in-game menu, RTC, or save states, and the X3 has no in-game menu, RTC, save states, and you have to reboot the flashcard in order to save in games.
As to where you can buy a console, you can find Gameboys and Gameboy colors all around ebay and such if you want an official one, they're usually pretty cheap (depending on the color and version), most Gameboy Color's I see go for around $30-$50 which is what I'd recommend personally.
If you want a clone, there's the GB Boy which is one of the more recent, "good" clones that have backlit screens (IIRC). Those are ~$50 as well.
