Kitty String are available now at YoYoWareHouse store based in Hong Kong.
If you're looking for specific yoyo string type and colour, you could visit directly to their store at Shop 216A, 2/F, IN's Point, Yau Ma Tei, Nathan Road 530-538
For more details visit their instagram account @yoyowarehouse
For other source of Kitty String product.
The String are also available here.