Here are a variety of ideas for sorting by shape. I have arranged these in order of the way I think they should be taught. Although usually real objects should come before pictures, I used photos of real objects first so that we could do the activity at Large Group time. Next, I have children work on sorting real manipulatives, and then we move on to paper shape sorting.
Shape Photo Pocket Chart Sorting
Children sort these shape photo cards into the different rows of the pocket chart. We do this as a Large Group activity, so we are working together as this skill is introduced to the children. I had originally planned to create a printable for this, but could not find enough pictures for all of the shapes. I found this set on Montessori for Everyone, and rather than reinvent the wheel, I purchased the set.
Shape Button Sorting
These plastic buttons are found in most educational supply catalogs, usually in the art section with collage materials, and are inexpensive. They come in a variety of colors and shapes: triangle, rectangle, square, circle, oval, diamond, heart, star, octagon. I use them for math because they are great for sorting by shape. The children sort each shape into clear plastic punch cups.
Sorting Plastic Jewel Shapes
These plastic jewels came from a craft store can be sorted by shape as well as by color.
Sorting Pictorial Paper Shape Cutouts
After the children have had practice sorting manipulatives by shape, we do this activity on paper. Children sort the paper cutouts by shape and glue them into the 4 sections on the paper. I used craft punchers from a craft store to make the cutouts. Of course, these are pictorial shapes, not the basic geometric shapes, but this is a good introduction to young children for sorting things by their shape. You may want to use a printable sorting sheet.
Sorting Geometric Paper Shapes
To make the paper shape pieces, I printed paper pattern blocks from the Math Their Way site. I printed them all the same color so that children would be sorting by shape and not by color. Children glued these onto a sorting sheet. These are great for saving for portfolios.
Classifying is a skill closely related to sorting. You might also like these Classifying Mats, available in my shop.