First up, this really isn't the Diamante you want, its got faded paint, 188k miles on the clock and a slushbox automatic, but in a nationwide search for Diamantes, I could only find examples in the rough. Think of this feature as a placeholder for future folks to drop freshly found Diamantes in the comments...heck, you can start now if you see a better one for sale.
The Diamante wagon was only available in the US with a 12-valve version of the 6G72 3.0 liter V6 pushing out 158 horsepower into a 4-speed auto. According to wikipedia, the 6G72 was also available in the:
1990–1999 Mitsubishi GTO (aka Mitsubishi 3000GT, Dodge Stealth)
1989–1995 Plymouth Acclaim/Dodge Spirit/Chrysler Saratoga
1987–2000 Dodge Caravan/Plymouth Voyager
1990–1993 Dodge Ram 50
1988–1993 Dodge Dynasty
1988–1990 Dodge Raider
1991-1996 Dodge Stealth
1990–1995 Chrysler LeBaron
1990–1991 Chrysler TC by Maserati
1989-1990 Chrysler Town & Country (early 1989 models only)
1988–1989 Chrysler New Yorker
1990–1993 Dodge Daytona
1992–1994 Dodge Shadow ES
1992–1994 Dodge Shadow
1986-1992 Mitsubishi Debonair
2000–2005 Mitsubishi Eclipse
1999–2003 Mitsubishi Galant
1988-1990 Mitsubishi Sigma
1990–1998 Hyundai Sonata
2001–2005 Dodge Stratus/Chrysler Sebring Coupe
1990–2002 Mitsubishi Diamante
1991–1996 Mitsubishi Verada (Australia)
1993–2001 Mitsubishi Magna (Australia)
1995-1999 Proton Perdana
1990–2006 Mitsubishi L200
1990–1996 Mitsubishi Mighty Max
1988–1997 Mitsubishi Pajero (aka Montero/Shogun) (Japanese and Middle East markets) 1997–present Mitsubishi Challenger (aka Montero Sport)
1994-2007 Mitsubishi L400(aka Space Gear)
Wow, that was a long list? Why was it important? Because of the first car in that list -- the Mitsu 3000GT/Dodge Stealth uses the same basic engine, and that is indeed a good thing.
Pick up a cheap used Stealth/3000GT like this 1995 3000GT here on eBay for $2,588 buy-it-now and rip out the 3 liter V6 and 5-speed gearbox. Swapping it into the Diamante should be a bros 'n' beer weekend or two. It basically builds itself.
If you live somewhere free from draconian eyes of the smog lords, you can pickup something wacky like this 2.0 liter 4G63T with 5-speed and make a few more horsepower with some bolt ons -- better for burning up those front tires. Find it here on eBay for $2498.
However, if you are really serious about performance and love starting impossible projects, this guy will sell you the twin turbo 3000GT engine and you can probably buy the transmission, drive shafts and rear end to create a VR4 Diamante. Find it here on eBay for $2,400. You'll need at least 64 cases of Monster energy, a lift, and $10,000 in miscellaneous trips to your local AutoKrazoneBoys to complete the swap.
See another swap candidate ready for your time/effort/money? firstname.lastname@example.org