there is a company on ebay that sells them, i remember looking before but it was about 100 quid for one mat 6ft x 4ft and that was 4 inch thick.
To be honest anything under 4 inch thick is probably not worth it for this sort of activity.
If you're gonna use a mat to learn to flip I would seriously recommend having a friend "watch" for you which basically means if you're not gonna bail but miss the mat he/she might be able to either move the mat under you at the last second or even push you onto the mat.
Also remember your whole body is in motion when you go into a flip so i would suggest your final jump/bounce on your approach be about 1-2 feet from the mat otherwise you risk clearing the mat altogether and falling on the ground.
Use some common sense and take it easy