Audball

My 2004 Mazda3 2.3L Sport sedan is going to need a new engine soon. I bought the car about a year ago without taking it to a mechanic...big mistake... I had a problem with the transmission, so after fixing the tranny problem my local mechanic suggested an oil change. He showed me the used oil filter, which was full of silver metal shavings, and oil didn't register on the dipstick....
The car has burned oil and been noisy ever since, but now the knocking sound is getting louder and I'm quite sure I'll need to replace the engine soon. My mechanic is quoting me approx. $1500 to swap an engine (labor only). My question is, where is the best place to get a "new" engine. I see several websites that have used (50 - 70k miles) engines available for $1800 or so....
Would like to keep my overall cost below $4,000.00 if possible.... thanks for any input!