The John East ones are quite nice but they are just one size, a bit cheaper at £7.75ea. They are here: http://www.east-uk.com/index.php/all-products/knobs.html
Another option might be Sandberg. I've had a look through my spares box and I only have 4 black knobs off a Sandberg bass that are not being used: https://www.cottoncontrol.com/browse/sandberg/de,0,308,0,0,20.html
They are on p2 a bit down the page. Mine are the slightly smaller ones judging by the published dimensions.