Exactly. I did the cheapo route when I bought a used T-56 off eBay that was supposed to be good. I paid $1100 for it and had to buy the clutch assy.
From day one it would jump out of 3rd gear, it would whine in 3rd, 4th, 5th and 6th gear and it was super hard to downshift into 2nd.
I recently replaced it with a rebuilt unit from Rockland Standard Gear in NY. They replace all the know weak parts with the right stuff. They sell it for $1800 plus a $750 core charge.
I ended up selling my old one on eBay for $900 in as-is condition.
I got a complete stock Corvette Z06 clutch assembly including clutch, PP, T.O Bearing, Pilot Bearing and new flywheel for $320.
No it was not cheap but it finally feels good to drive. I don't have to keep my hand on the shifter when it is in 3rd anymore. Sure makes track days easier.
As for your question about which vehicles to get them from, all the 6 speed F bodies with the LS1 motor from 1998-2002 should be fine. The earlier F body 6 speeds had a different bell hosuing.