I think White Sewing Center still has the most reliable quality for the Supermatics, at least he has the best reputation. If you are in the US it's proably the best source. There are other suppliers, but some new replacement pulleys are not worth the effort. The original pulleys had a bakelite core to support the rubber and ran very stably, some of the newer ones turn almost inside out when ran on higher speeds.
I managed to get my hands on an aluminium pulley, and if you can it's worth it. Now and then a guy with a lathe machine and know-how makes up two or three, and he might be persuaded to sell one. They are fitted with o-rings and have good grip. I found mine through the vintage elna group on yahoo.
A temporary solution can be to sand down the rubber pulley to even out the flat spot. You might get better grip that way too, since rubber is often in better condition under the surface. Some have used violin bow rosin on old pulleys to give them better grip again.
I remember mine required a lot of oiling and cleaning before it ran as it should again.