Hunt told me the seals aren't available separately, and I would have to buy a pair of end caps which come with the seals, which seems like a waste since the end caps themselves are fine.
I'm sure they must be a standard industrial seal, but I can't find anything similar. They're a flat seal with a rectangular profile to fit in the slot on the end cap and an angled lip:

A bit like a V-ring, but the lip is on the outside of the seal rather than along the axis.
Anyone know what this type of seal is called?