We have a LeapFrog Imagination Desk and my 4 year old loves it.  But we go through the workbook sheets like wildfire!  Then I found out that you can print replacement pages, activity sheets and coloring book pages for free at LeapFrog's website.  You just need to download their print utility, and it works like a charm!  You do need to have the cartridge that came with the original coloring book to enjoy the audio, but two workbooks don't need cartridges: ‘Learning Letters with Leap’ and ‘Let's Learn Letters.'