Where to buy good used engine?

I have a 1999 silverado with the 4.8 and it has the bad castech heads and is leaking coolant into the oil, so, i am looking for a good quality used engine to drop in. Want to go with a 6.0 to avoid this leaking head problem again that the 4.8 and 5.3 could possibly have. Anyone know of a trustworthy place to buy a good 6.0 thats reasonably priced and will ship? I ordered one from AM used auto parts in tampa fl. but cancelled the order after seeing they are flagged on the BBB website and ripoff report.com. I thought it was too good to be true with the low mileage and price they had. I need to do something fast as this is our family vehicle.
Thanks

What I did was watch in the paper(ads) asked friends,phoned around to wrecker yards(that were close) and bought a whole truck with a Ls 6.0 and a 4l80e tranny in it !!! It had around a 100,000 miles on it but these motors can go alot further than that. Being a picky person though I opened it up and cleaned it up (changes gaskets etc) and had the tranny gone over by a friend and I was ready for my swap( that still hasn't happened) due to lifes little problems coming up !! I then sold some parts off of it and when nobody wanted anything else sold it to the scrap yard again. In all I was out only a few hundred dollars and had what I wanted !! Just be patient is about the best advice I can give !!

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