I've bought bobbins in the UK, both with and without the J, and and several boxes of coloured ones from China (useful for different threads - cotton, polyester, embroidery, specific bobbin thread etc, plus different weight threads). The occasional one or 2 of the Chinese ones have too small a central hole to fit on the winder but apart from that I have had no issue with any of them. I've also reused some of the pre-wound bobbins after I've refilled them and again no issue - plus I've used bobbins from my old Elna Diva at times when they have the right thread on them.
All work fine in my 15000
"Great dancers are not great because of their technique, they are great because of their passion" Martha Graham