Looking at the .303 rifle bullets available, I found this one
but apparently, other bullets made for the SKS are also suitable despite a nominal diameter of .310. That includes these
The slightly smaller diameter shouldn't be a problem. I've shot .308 bullets through my 303 Brit and had good accuracy. The weight of these is troublesome. They are short and fat and without the oomph of a necked cartridge, their range won't be great. It would be nice to get a lighter faster bullet, but that would mean shorter and that's bad for accuracy. I think a saboted .17 would be a better option.