www.powerpro2000.com (3.5 Isuzu longblock)
www.rpmrons.com (3.5 Isuzu shortblock)
www.car-part.com (clearing house of salvage yards)
Isuzuparts.com, Isuzuparts.net...there are lots of options & these are just a few. Do a forums search for more.
And yes, any 6VE1 should bolt right in (from a Trooper)