I've always been a fan of fleece onesies, personally, but they seem harder to find these days.
The one I have currently is from Cosyndry, which I've had for a good 4 years and it's held up fairly well considering I haven't exactly been gentle with it washing wise. Looked to see if they still had the print, in case I ever needed a new size or something, and they just straight up don't have fleece onesies anymore. You could probably request a custom made one, but they don't make them readily like they used to.
Cuddlz is the only place I've found that has a decent variety of fleece onesies now, but I've never tried anything from them before and some of their products just look cheap to me in the pictures. The onesies aren't helped by using pictures on mannequins, but they have some variants with pictures of models and they look better in those. I might try one just to see how it is, they actually have fleece mittens/booties that match some of the onesies too so that's interesting.