A few suggestions; try Pacific Rim Cameras, in Oregon, USA.
Keep checking with them, as inventory rotates through their doors constantly.
Attach the holder to your existing flash with strapped Velcro.


Example of a Vivitar diffuser kit:

95790 VIVITAR LENS KIT FOR 283, COVERS 24, 28, 70 AND 135, IN PLASTIC CASE AND WORN BOX $20.00
http://www.pacificrimcamera.com/images/95790.jpg


Example of a flash, filter/diffuser holder from Vivitar:

87665 VIVITAR LENS FILTER HOLDER FA-3 FOR 2500 FLASH, ADAPTS FILTER KITS FK-2 AND WFK-2, NEW IN WORN BOX $8.00
http://www.pacificrimcamera.com/images/87665.jpg


A Rollei Flash unit with Rollei diffusers:

138165 ROLLEI BETA 5 FLASH, WITH WIDE, NORMAL AND TELEPHOTO HEADS, AND COLOR FILTER KIT, WORKING, LOOKS EX $95.00
http://www.pacificrimcamera.com/images/138165.jpg