I had a similar problem when sourcing some for the East Midland's group of the GSS layout.
I needed quite a few as all the pointwork was being relayed and I thought clamps better than fishplates for future maintenance.
One source was Split Jaw http://www.railclamp.com/ , take note according to them there are three types of code 250 rail, Peco has a 4mm foot. so Split Jaw call it code 250-4.0. I chose not to buy from them as they use a different size of screw to the old Hillman and I didn't want the complication of two similar looking but critically different sizes of Allen keys.
I decided to buy Uncle Herm's code 250 clamps despite the fact that they needed to be machined down to fit Peco rail as they used the same screws as Hillman.
A mate was more than willing to machine the lot in exchange for a bottle of Gordon's!
However I've just checked their website and it states
" Due to a lack of demand for code 250 joiners and the ever increase differences between the rail base dimension between brands, we have reluctantly decided to stop producing them. We will be selling our remaining inventory on Ebay (Search for "CODE 250 TRACK JOINER"). We will consider making them again, if someone places a big enough order. "