There are only a few artists that I've bought more than 1 of their albums/CD's over the last 25 years that have not been previously mentioned:
Johnny A
Ryan Adams
Neko Case
Rosanne Cash
Lloyd Cole
The Delays
Mark Eitzel
I Am Kloot
Ivy / Fountains of Wayne
Jenny Lewis
Shelby Lynne
Aimee Mann
Nada Surf
Kim Richey