When I built my pole barn, 24x36 feet, I first did the usual search for "pole barn plans" and found a wealth of information. When I decided to go with trusses and started looking for materials and such, I found that all the major suppliers of materials were more than happy to draw me up plans and materials lists.
I'd start there and get a really good idea of what I needed and then ask for specifics. Some of the plans I found were at university extension sites. They were pretty detailed, but some were old enough that pre-built trusses were not mentioned. I ended up more-or-less designing it myself with hybrid ideas from several sources. If I did it again it would be much better.....of course.
Chuck