You scratch-build one, and as soon as you are finished
somebody will be sure to offer a kit

The Northern Pacific Railway Historical Association offers an NP standard
laser cut scalehouse in HO on their website.
You can go to www.nprha.org and go to the Company store or this is a direct
link to the kit. (Sorry if the link wraps.)
http://www1.storehost.com/stores/xq/xfm/store_id.615/page_id.23/Item_ID.134779/parent_ids.0,0,1/qx/store.htm

Dan Stinson
Helena, Montana