Most middle of the road model railway magazines, since year dot have included at least one plan of locos and rolling stock. Railway Modeller used to include a loco drawing, wagons and lineside drawings each issue. I don't know about Britain, but here in New Zealand most model shops have the odd carton of old magazines with something like "20cents per magazine" marked on the side. Scruffy second-hand bookshops get the odd collection so a modest waving of cash will get you a whole carton. If you decide on a particular prototype someone will probably be kind enough to scan their drawing for you.
There were, and probably still are, collections of drawings from draughtsmen offered for individual sale. (the name Roche springs to mind) Someone in the UK will probably tell you about thiose.
I'm still looking for drawings of LNER ferry wagons from the 1923-30 period, but nothing has come up so far.
A lot of Ian Beattie's drawings for RM have been collected in book form (the locomotives to scale series), but I don't think you get the accompanying text from the magazine articles.
I advise a trip to your local library, or browse the catalog online if it's available. It doesn't cost a great deal for an inter-library loan from other counties if they don't have what you want. Find out if there's a county store where the non-shelved books are stored. The one in Aylesbury (for Bucks county) is a treasure trove of Railway books, compared to most branch libraries.
There are also "Skinley" drawings available from:
The Sussex Modeller (Phil Brice)
20a Rowe Avenue Peacehaven East Sussex BN10 7PF tel: 01273 581 265 e-mail: email@example.com 'Skinley' range of drawings, also LFC nameplate range