Regular (+1, +2, +3, +4, +5 and +10) diopters are available from low to reasonable price.
Special (+1/8, +1/4, +1/2, +3/4) diopters are available for much larger diameter lenses (so you'll need to step down) at prices ranging from a few hundred to twice that much, so that's probably cost prohibitive.
Schneider Kreuznach (B+W) makes high quality products, but the diopter you want is special order and for larger lenses used for professional cinematography.
A webpage with photos demonstrating the magnification effect of various regular diopters is available here: "Using Diopters - Close-Up Lenses".
Your difficulty is described in the DPReview Forum thread: "Help with close up zoom on Nikon Coolpix P900!" where Stephen Ingraham explains:
"As noted in other responses, the further you zoom in, the further you have to be from your subject for the camera to focus.
At full zoom (2000mm equivalent), you have to be 16.5 feet from the flower for the camera to focus.
At half zoom (1000mm) you have to be about 9 feet from your flower.
At 500-600mm equivalent (1/4 zoom) you have to be about 6 feet from your flower.
If you are closer, the focus will try ... but you will get a red square where the focus square is in the finder. That means the camera has not focused.
If you are the correct distance, or further, you will get a green focus square.
If you zoom while looking at the finder or the LCD, you can see were it goes out of focus when zooming in. Past that point it will not focus no matter what you do.
If you want to do close-ups, set the camera in Close Up Mode in the Scene modes ... it will zoom to the widest angle. That is where you can get closest to your subject. As you zoom in, you will see that the zoom indicator at the top of the viewfinder is green unit about 1/5 zoom, then it goes clear again.".
I think you are going to have to be satisfied with +1 diopter, use of the close focusing mode, reduced zoom, and some cropping; unless it's worth over a few hundred dollars to get these photos.
Be prepared to scare off a few bugs if you have to get the camera close and want to move around a lot. Placing the camera up close to a prime patch of flowers where butterflies land and triggering automatically (with a seperate trigger) in your backyard (over a period of hours or days) may be the only way to do it on the cheap.
Surprisingly bees are quite friendly and allow you to get closeups, if you'd be willing to switch from butterflies. This bee photo was taken from about 6" away, handheld.