Check out this seller's items on eBay - numerous choices of "faux leather for costumes," etc.
The fabric in this link looks very supple and drapes well - it comes in black and brown and the price is right, $6.99 a yard, and shipping is only $3.50.
A major feature of this (and most) faux leather yardage is that the fabrics are typically 60" wide. You'll need much less total yardage with this wide width because faux leather typically doesn't have a grain, you can cut your pattern in any orientation.
Real leather is actually quite stiff and doesn't drape nicely - unless it is suede or lambskin which are both super expensive.
Since you say you are making a costume, I assume it won't be worn everyday (but you never know!), so I wouldn't go to the trouble to try and retrofit a used leather garment or piece things together. Real leather is difficult to sew, either by hand or by machine, and as it ages it gets quite stiff.
Faux leather - as others have indicated - is also an ethical alternative to real leather and you can rest easy knowing that only two or three Polyesters were sacrificed for your costume.
Note: be sure to practice on a test swatch before you start sewing your actual garment. That will help you get your technique down and let you get a feel for how the fabric handles.
Good luck! Sounds like a cool project.