My TF came with MG Centre hoops & windstop (I think). The fitting's a right lash-up - cracked T-bar, mess of different screws, nuts, washers, etc. and missing central perspex section locking screws (black plastic knurled thumbscrew heads with M4 threaded bolts). So a web search for knobs ensued - amazing what you find! But the most interesting was a pair of stainless steel thumbscrews from Acculimited, who advertise on eBay. LOADS of fittings. Mine arrived really quickly, with a friendly service. Highly recommended and if you've got trouble with your knobs you know where to go!
eBay posting is:
M4 x 20mm Knurled Thumb Screws (DIN 464) - A1 Stainless Steel... Pack of 2
Pic attached (hopefully)