Here are a few links to Elmo costumes for people of all ages. The cutest has to be the baby costume below - just click on any image and you'll be taken to the store that stocks the costume so you can check price, availability and sizing.
The Elmo costume above is ideal for toddlers while the Elmo costume below is for older children - just click on the images to be taken to the stores that stock these Elmo costumes.
Here are some more Elmo fancy dress options straight from Amazon.com pretty much the most trusted online store...