The materials ended up incredible easy to find. Make your own for about $5.
o Cup with a "lip"
o Rubber Band
o "stretchy" cloth.
I forgot where I got my cup, but I used an enamel baked steel cup. Get a quality one, where the enamel can stand some dings. Rubber band was either from a news paper or grocery. "Stretchy" cloth from the scratch/left over section at Walmart, costs < $1 and can make several covers.
Step 1: Making the Lid
Place the 2 squares together. With one square on top of another, overlap the 2 pieces by about 1".
For convenience, I sew the 2 together on the edges.
Step 2: Assemble
Step 3: Fill
He has been using it for several weeks now. It has survived several drops, but no spills. To wash just disassemble. Make more than 1 cover so that you will always have a clean dry one at your disposal. Place stickers on cup to personalize and make it their own.
Cheap, eco-friendly, healthy, lasts forever... After they grow older, reuse the cup well as a cup or another neat instructable.