Over-tanning is a concern when using Melanotan-II. Once you reach your desired level of tan, you'll have to use a very high SPF sunscreen whenever you go outside, because believe me, your skin will turn brown if you don't. I hope you know that you'll have to keep using M2, you can't just stop once you get your tan either. Side effects can be a b**ch as another user stated, but some people can manage, some can't.
As for sources, I believe AllAmericanPeptide.com carries M2, but you'll also need syringes and BAC water.
I used to get mine from a website called melanotanmagic ( I'm UK btw ) This is going back 5 years ago when it was popular. Used to come in a vial of powder that you injected bacterial water into. You will have more luck getting it from sites that sell steroids, as bad as that sounds.