The Bi Flag is a visibility symbol inspired by the Rainbow Flag, and in turn it inspired the Trans Flag - it was thought up twenty years ago in 1998 by Michael Page who used the colours of existing bi triangle symbols and made them into oblongs. Hey presto - flag!
Page published the design on the now defunct Bi Cafe website and it has spread worldwide from there - it's the most successful bi symbol ever.
There's a great history of the flag over at Biscuit: A Brief History Of The Bi Flag
As a visibility symbol the flag is unparalleled because it's so simple - three colours with no shapes, just as stripes. It can be evoked with any three objects of the right colours, much like the Rainbow and Trans flags.
This is incredibly important to signpost our identities to each other, and allow bisexual people to find our communities.
As Page's original Bi Flag site has long gone, here's some images of the flag in different resolutions and shapes you can use, and some variant ones too.
You don't have to use the exact colours - and they can be tricky to find, but any three stripes in pink, purple, and blue will work. For the historic colours, see the Bisexual Pride Flag on Wikipedia. Remember that the purple stripe is narrower!
This is the colours most commonly used for the Bi Flag - Bisexual Flag
These are slightly lighter, good for some uses like phone wallpapers - Bisexual Flag But Slightly Lighter
These are darker - Bisexual Flag But Slight Darker
No, that's the thing about visibility symbols - they're deliberately free for anyone to use. Gatekeeping visibility works against the whole idea.
These colours? Of course - they're just colours. What makes them the bi flag is the meaning we ascribe to them as bi+ people.
These actual images? Of course - they're simple geometric shapes. Even if it wasn't against the point, anyone trying to copyright them would be laughed out of court.
Well, no-one. Public domain means public domain. Yes, they're making money but rather than worry about who is selling them, appreciate who is buying them - bi+ people! For many activists the first place we go when we want a lot of bi flags is Amazon or Ebay - making physical flags is time-consuming and expensive. We're happy to let someone else make the tools - we need them to fight bisexual erasure. And besides, well made cheap bi flags worn en masse as capes at Pride events are so totally inspiring to see!
If you're writing about the history of the flag for a school paper or an article then sure, mentioning Page as sole creator is the right source but no-one needs to do so just to use the flag for their own projects, non-profit or profit.
No, no need. Anyone who tells you that you do is trying to prevent visibility, and is working against the whole point of a flag.