Ah thanks FTT 
The reply they got from ZSA is worth repeating here:
You can use standard Cherry MX-compatible keycaps. The ones that come with the Moonlander are all OEM Row 3; the material is PBT, and they’re made by Tai-Hao. All the keycaps in the main sections are 1u in size, the “piano keys” are 1.5u, and the large red keys can be replaced by 2u keycaps.
Those Jelly keycaps look nice too! Tho I reckon they’d look better on the white version 
Which colour did you order btw? I went for the white…