Are you looking for the base seat fabric or a custom canopy/seat liner?!
The only place to get the actual seat fabric is eBay, since the Frog has been discontinued for a couple years now. Unfortunately, the Bugaboo site doesn't even carry it anymore.
For custom canopies & seat inlays/liners...
[We had a Mama-La for the original Bee, awesome quality...not sure if she's still creating though]