Tichy Train Group

Walthers carries them

Thanks. Are there any other

Thanks. Are there any other places where I could buy walls, doors, windows and roofs?

RSlaser Kits

If you are in N scale, RSlaser kits carries a wonderful line of scratch built items.

https://www.rslaserkits.com/N-Scratch-Build_c_38.html?viewall=1

They also have some interesting bits in HO scale as well.

https://www.rslaserkits.com/HO-Scratch-Build_c_34.html?viewall=1

Woodland Scenics carries the DPM line of modular parts for structures in N, Ho and O scales.

https://woodlandscenics.woodlandscenics.com/show/category/ModularStructures 

Grandt Line also has a series.

https://www.grandtline.com/

A couple more options

http://www.rail-scale-models.com

https://rixproducts.com

For plastic / styrene Tichy

For plastic / styrene Tichy Train Group https://www.tichytraingroup.com/, Plastruct , Evergreen  ,

for wood Northeastern https://www.northeasternscalelumber.com/,

Metal castings ,Wiseman http://wisemanmodelservices.com/ .

For scratch building on the cheap... I would suggest you pick up a copy of"Easy to Build Model Railroad Structures " usually you can still find copies on ebay for $5 - $10 US. 

You'll also want to pick up a scale rule or print one out and laminate it with packing tape. https://www.printablerulers.net/category/hobby

HO Scale?

https://sanjuandetails.com/architectural-parts/ho-scale/

San Juan bought Grandt Line parts when they closed up shop.  They only sell online as far as I know, but their website is above for HO scale.

I just 3D print what I want

I just 3D print what I want and need