I haven't dealt with them personally, but NY Findings has some on their website - 15 mm 18k white (also 15.5 mm and 16 mm, in platinum, 14k and 18k yellow and white gold):
http://www.newyorkfindings.com/Clasp...One/Page54.jpg
To enlarge the picture, click on the lower right hand corner. There may be others - they have a lot of stuff on their site!
Perle