Here:
Day Tripper
We have used them several times (and are going again on Thursday this week!)
The link takes you to the P&O booking site where you can book at £19 return. Print out your reference details and then just turn up.
Once there, we usually head to St Omer (about a 15 minute drive) where there is a large Auchan shopping centre and it is much nicer that Calais itself.
Hope this helps someone!