What is a flash cart & do I need one ?

Flash carts allow you to test your custom GB code on a GB. You can also use them to play commercial GB ROMs that you might obtain from the internet but this is illegal. Especially if the commercial ROM is still available for sale.

Flash carts are more ideal for testing custom code than gb emulators because emulators often allow you to do things that real GB hardware does not allow due to hardware limitations.

Where can I get a commercial flash cart ?

There is currently only one place that publically sells flash carts and that is Bung Enterprises Limited. Nintendo sells a 32mbit flash cart (one with & one without rumble support) but this is only available for sale to licensed developers. The Super Smart Card made by China Coach Limited is no longer available and the company itself is out of business. The C3 cart made by Jeff Frohwein is no longer available.

Here are some links to Bung product distributors.