The Mini is fairly new and as such the after market options will be limited. Ebay is your best bet. Also consider the Cube, wich can be had for a very reasonable prices and is also small with a nice upgrade path too.
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PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...