I saw this fabric button tool kit from a Taiwan website (http://grass-es.blogspot.com) and the price is affordable. She has size of 18mm, 22mm, 27mm, 38mm. It is RM5.2 per set with a few button sets included.
Fabric button tool kit is also available from local sellers:
There are also small quantity refills from:
1) 28 mm - http://hong2lee.blogspot.com/2010/08/button-f130-fabric-button-casing-28cm.html
2) 13 mm - http://kawaiistations.blogspot.com/2010/07/button-mold-13cm.html
3) 1, 2.5, 3, 4 cm - http://heart-handy.blogspot.com/2010/07/wrap-button.html
But you could also make fabric buttons without a kit. Here is a link to a tutorial:
Thanks to the link owner for sharing this tutorial.