One is a passenger-only ferry; and the other is one you can take your car over on if you choose to. You can always leave it in Port Angeles no matter which ferry you take. The links should answer your other ferry-related questions.

As for buying fireworks...in my area, there are fireworks stands all over beginning about two weeks before The 4th. They are typically in grocery store parking lots and other visible areas along the main roads so you can't miss them. Most of these are run by various community organizations, like the Lions, Rotary, Elks, Eagles, etc. There are also a few stands that are private, business interprises. I have a feeling this is typical in most smaller towns so you shouldn't have any problems finding fireworks for sale. If you can, try to buy it from a group doing it as a fundraiser. I know some of our local organizations make a huge chunk of their annual budget via these sales so it really helps them out. To find them, I'd just ask a local person at the gas station, a restaurant, etc. I'm sure they can point you in the right direction.