Uh, when you say "models" you might wanna specify the software you use. Blender or some other extensive 3D modelling suite? DAZ? SFM? MMD? Illusion character creators such as Koikatsu or Honey Select? Or do you just mean reference images for 2D art?
And you'll want to clarify "undeveloped" too. Canonically Megumin has a certain body type. Not prepubescent as per rule 7 on here, she clearly does have breasts and adult art could easily show her with pubic hair. But she doesn't look like an hourglass bimbo. On the other hand a lot of art does straight up turn her into a flat-chested loli, so if you only want to exclude that, fair.
As far as images go Rule 34 (both XXX and Paheal) have plenty of her. Pixiv has more loli-leaning stuff mixed in. E-hentai has image sets and the like. Out of those Paheal is the only one which doesn't have content tags to refine your search e.g. "large breasts". Since this largely is a matter of taste going through yourself is a much more productive approach.
