There's not really much of a discount anywhere, IMHO. Sometimes you may find a package deal with a discount but otherwise I would buy them from undercover tourist with the code you get in the mousesaver's newsletter. Will make it a few dollars cheaper but that's really the only discount I know about. Have fun!
If you have a AAA office close by you can drop in and pick them up but you're saving something like $2/day. If you get them to mail them out to you that kills your savings right there.
I saw a nice deal on mousesavers for buying tickets through the Florida Hospital and then the money goes to charity! Not much savings, but a good use of the money and a tax deduction.