I worked at a bikeshop for most of highschool/college. We saw quite a few that were used by special needs kids and McDonnell-Douglas had quite a fleet of them.

There are a couple brands out there, but Worksman was the most common ones with what seemed to be the best support. Recreational Cycles

I wasn't a big fan of them. Riding them was kinda like driving a dumptruck but I really appreciate the fact that they open the door to people that don't have the ability to ride a bike and they are a very effective way of covering a large area like a hangar without using gas but they are a PITA to work on. Lots of specialized parts, difficult to transport, and heavy. One of the reasons we got a reputation for being the authority on them in St. Louis was because one of the guys I worked with had a special needs child, he was also a wizard when it came to adjusting the Sturmey-Archer three speed hub. I spent more than a few hours cussing while trying to build/repair these for customers. Most of the time it involved after-hours work because they took up so much space.

I always advocate wearing a helmet when riding. They've saved my life on a couple occasions (hit by a car and hit a tree head first at nearly 30mph and walked away from both) and I've seen first hand how bad it can be when you're not wearing one.

Take the lumps and learn to ride a real bike. You'll be happy you did. You'd really be missing out on some beautiful scenery in the Seattle area if you limit yourself to a trike.