Yes you are in the right place for anything
related to HO trains and equipment.
I didn't find any plans or templates but here
are some ready made farm buildings.
http://www.hobbylinc.com/cgi-bin/s8.cgi?cat_s=UGC&tag0_i=949
Maybe you can use them as a guide to build your
own. Most hobby shops, including Hobby Lobby
and Michaels sell scale balso and basswood
lumber that you can
use. Some Hobby shops have scale building
materials of styrene plastic that is designed to
be roofing, or side walls of various styles.
You can find very economical hobby paints
in the Walmart Craft section.
Don