I got ours from ace hardware, i don't remember the name of it but whats great is that i don't have to use those round spools of string, that seem to not work, there is an attachement or something, that lets you slip in peices of string they are pre cut but you can cut your own which would make it alot cheeper, I don't weedwack often i hire it out due to the amount that needs to be done.
I spent several hundred on ours because I like power!