I like wish. It's an app for your phone but it takes forever to ship. They have wall decals and other things. Really cheap too. Read the reviews first tho.
I got some of mine from hobby lobby on sale (use 40% off online coupon if it's full price!) and downloadable art from etsy ($5 each, printed off at costco for cheap!) that I framed w "at home" frames.
Babies r us usually has a small section of wall stuff .. I didn't love all of it I got two things from there though. Check bed bath n beyond, Christmas tree shops, tj max etc. I'm having a hard time. Still got one wall I might just do shelves on cuz I can't find that many things I like... and my daughters room is white gray pink and lavender. But her walls are white Bc were renters and the landlord doesn't want us painting. I had to think outside the box because it's hard to find all baby themed stuff (at least in my area) and so I got one really big picture of pretty flowers in her colors on canvas ... wasn't my first idea but it fits and is nice and big on the wall so it works !