As Stu says above, Reba’s only come in 100mm flavour now. You’ll need the Revs for anything more.
Classifieds and some patience is now your friend. Plus, thinking about it, most of the 120mm Rebas (if not all) were 20mm in thru axle versions.
On One had some cheap ones earlier in the year and i missed out on them by days and ended up tracking some down from 29squadron for my Honzo.
Wish i’d stumped up a bit more for Revs though (especially as there was (is) such a problem with offset legs on the 20mm maxle Reba versions (so you can’t run 2.4″ tyres in them). My older std steerer 120mm versions are also 20mm.
Manitou might offer something but again, i’m not sure in a 15mm option?
Are your hubs interchangeable for 20mm as well as 15mm? Most newer hubs are (Hope certainly are and they’re very popular on here).