I make them myself. I use screenshots of characters and cut them out to remove background.
The ones from Kirby's Return to Dream Land are provided by my friend Эллис Элдебри. She uses Dolphin emulator to take screenshots from otherwise impossible angles. The notable exception is Goriath's page as I found his renders from internet.
The ones from 3DS games are taken from videos made in Citra emulator. I just screenshot the scene and then use the screenshot as a base to make a background-less image.
The ones from Kirby Star Allies are just taken from various Star Allies videos (e.g. Rick, Kine and Coo's images were taken from video showing all title screen animations, Mr. Frosty's and Bugzzy's images are taken from Taranza's trailer, etc.).
Both of you are doing a very beautiful job with this. The main wiki that I use is the VS Battles wiki and since I found this images I've been replacing some of the ones we have for Kirby characters with the renders you guys were making, all of them look pretty cool.
I'm sorry, but there were no good videos or screenshots made in Citra which feature pink Kirby ghost. I may try to cut him out from 3DS video instead, but the image would be very pixely at best, thanks to the fact that every video made in actual 3DS ends up looking very pixely.