I'm doing a dr. seuss nursery too. I almost got the ikea korall mobile that PP mentioned but it was too small for what I wanted. I ended up choosing to do a DIY mobile using a 10 picture mobile off amazon and some felted letters from etsy. I haven't done it yet but know it's going to be cute.
Re: looking for Dr. Seuss-y mobile...
Oh! Ikea has a great fish mobile that could be Seuss-ish! It was my first thought when I saw it that it would be great for a suess theme.
https://www.thingamababy.com/photos/uncategorized/2008/01/03/mobile.jpg