I don't know of a source other than if someone on here has one. I've seen cheekpieces for sale on their own just once or twice on egun in the last few years.
I'm not certain, but I think the first and last model SUs came with just one cheekpiece. The first pattern definitely had only one and the following pattern definitely had two. I have a feeling the final pattern SU, the one with full length stippling under the forend, reverted to a single cheekpiece - I expect someone can confirm or dispute this.