Yes, you must get your satellite from DirectTV.
No, you do not have to change out your receiver if you get an HD set. You existing receiver will work fine sans HD content, obviously.
Not sure what exactly you mean by “generic hd receiver” but you can buy a DirectTV HD receiver from places other than DirecTV. I’d recommend browsing around http://www.dbstalk.com forums.
Sounds like a monopoly that needs to be invaded. $$$