Arc Euro Trade sell T nuts though it depends what size you need.
I bought from them as they sell the machine I have but the ones I got were 8 mm with an M6 thread.
Depending on your application, these may or may not be suitable but, Steve Maskery sells "Flange Nuts" to fit T-track at Workshop Essentials.com. You may have to double-check the size of your track but, I think the price is right.