I find images in a variety of places. I don't remember where I've found all of them, but it's mostly a combination of maternity sites, porn sites, and various threads here. I just keep an eye out and stash images I might be able to use later. The larger the image, the better.
As far as the edits involving clothing, I treat them pretty much the same as those with an exposed belly. I try to stick to photos that have simple patterns and textures because it is much easier to find something that matches. If I haven't saved a belly that I can use I'll check different maternity sites to try and find one. Motherhood, Pinkblush, and ASOS are places I usually look. The photos from those sites are often too small to use for editing though, so I upscale them with Gigapixel AI. Try to focus on the texture/pattern and posing instead of the color. Color can be fixed pretty easily later. As an example, this is the belly I used for this Lindsey Stirling edit:
The first example you have doesn't appear to be too difficult. I would probably try to use the belly from one of these photos:
Use the Free Select Tool to cut out the belly. I usually make a selection from the underarm to the knee. Once you get it positioned in the right place, use a layer mask and a soft brush to blend the upper and lower parts into the original image. Then I'd sample the color from the original clothing and recolor the belly to match. (Same process as step 6 in the pdf.)
I'd also reduce the brightness and contrast to help it blend in.
The second example is much more difficult. Having a hand on the stomach or waist usually makes an edit look unnatural. When you imagine where a belly would be in this image, her hand would need to be shifted away from her body so that it rests in the proper place. You could do that by using the Free Select Tool to cut out her hand/arm, maybe all the way up to the shoulder, and try to rotate it into position. It's possible to do but requires a lot of effort to make the final result look realistic.
