http://www.ebay.com/itm/like/201290468803?lpid=82&chn=ps ?
Or get a 250/250 and a 500/500 from the same manufacturer, split them apart and combine the different halves so that you end up with two 250/500 pots.
There's also an official Fender 250k/500k stacked pot.
I wonder if you were to glue or solder the two together and grind down that top shaft to make it into one single knob, it a typical Tele knob would fit over and around the outer most ring.
Another approach to take that isn't quite as good with regard to the tapper, but would function similarly would be to put a trim pot between the center and pickup luck on the volume pot, which is only allowed to be in the circuit when you have your single coils selected, so that you can get a 250k pot's load while still having a 500k volume pot in the guitar. If you're the kind of guitarists who leaves the volume at 10 all the time, but wants the 500k load for a humbucker and 250k for singles, it would work perfectly.