The RhB publish outline drawings of their freight stock. You need to do a search for something like Datenblatt_Flachwagen_Typ_R-w_Serie_8082_-_8218 and you will find it in .pdf form. I've downloaded as many as I can find. Oddly nothing similar for passenger stock.
I have e-mailed MDS-Modell about purchasing bogie frames but I assume they were getting ready for Stuttgart so I'd hope to get something this week. For wheels in these I'm using Fox Valley 33" on 0.563" axles. Looks silly to me but it works!
For members of the N Gauge Society they do bogie kits and several are very close to RhB types, short wheelbase, coil controlled axles. I think they have the advantage of being plastic and so are modifiable more easily than Kato or MDS. £4.60 a pair + postage but you need to add wheels and couplers.
Cheers,
Mike Beard.