There are many ways to skin this cat, but the following arrangement tends to have the best long term results (though not the cheapest up front):
1) Figure out what you think you want. Basic style, size, rough layout, etc.
2) Hire an architect. A good one. Not an architectural designer, not a draftsman, but a registered architect who knows what they're talking about and is willing to take responsibility for and stand by their design. They'll take what you did in #1 and turn into a functional and good looking design.
3) Your architect will hire a structural engineering consultant. They'll design the structure - lateral force resistance against earthquakes and wind, beams, etc. They'll also develop the specifications for the trusses you need.
4) Once the design is done, it will go out to bid to qualified contractors. Not the low bidder, but the most qualified bidder within your budget (look for an itemized breakdown of cost - it means they actually took the time to look at the design and figure out how much everything will cost).
5) The contractor will send the plans to the truss plant where they will be designed. Then, a truss engineer will check the calculation output. Then, the EOR (the engineer who designed the house) and the architect will review it and make sure it was designed correctly and fits the house.
No matter how you do 1-4, number 5 really doesn't change. Structural consultants don't design metal plate connected trusses and truss plants won't build somebody else's design. You might find an engineer that will design plywood gussets, but they'll probably cost twice as much as buying them from the truss plant. These things are custom designed building components, and their economy comes from a) the truss plant's economies of scale and b) the optimization of the truss plant's process. Each one is a little different. For instance, if I need a floor truss with a bit of a slope in it for some unique profile, one of our two local truss suppliers can't do it. The other one doesn't have a problem with it. Just has to do with the way their tooled.