Well, here is pretty good way to fix that on the cheap.
It should last longer than any replacement insole you could come up with and be more comfortable than the original.
Note: Because this is a thicker material, you'll have to replace the insole in both shoes, the broken one and the matching shoe as well so your feet will be at the same height when you are walking around.
Step 1: Things you need
shoes that need a better insole
pair of good scissors
Step 2: Remove the thong straps from the flip flop
Step 3: Trace the outline of the old insole onto the flip flop
Step 4: Cut out the insole shape
Step 5: Replace the old insole
You might have to stuff it in a bit here and there, but it should fit snugly with room for your foot.