Hello Dark Knight,
There is a used 3-jaw 6 1/4 inch chuck on ebay for $45.oo (buy-it-now). It is rusty and needs jaw tops (it's reverseable jaw type). No back plate included and I didn't see any chuck key... but the price is low for chucks anyway. No guarentee that it is not worn. Brand unknown...
This chuck would be a salvage/resto project requiring a mill to make fresh jaw tops but it is rather inexpensive.
New econo 3-jaw 6 inch chucks (chinese) run about $130.oo less back plate. Top quality American 3-Jaw chucks this size cost several times that amount (as you likely know).