Trying to find an inexpensive costume for a 7 month old can be hard. I had this idea to make our son Nathan a turtle costume.
For his shell, I used an old bicycle helmet and painted a turtle shell pattern on top using craft paint. The helmet straps fit under his arms and fastened on his chest. The groove of the helmet fit his back and shoulders perfectly! He was able to move about and crawl around (looking just like a little turtle) with no problem.
After Halloween, he wore the sweatshirt and pants for a number of months and we actually have kept the helmet incase we need an extra. In the photos attached, you'll see him receiving his prize for the best costume!