Ive done the swap, well 4.3 to 5.3 anyway. The motor mounts were a pain in the ass. Theres 6 different mounts if im not mistaken, and i went through a combination of them before i finally got the motor sitting right in my truck. I do have a some leftover mounts if your interested, not sure if they will be the ones you need or not though.
I have a brand new v8 4wd passenger mount, and a used v8 2wd driver side mount (the style that bolts on top of the frame, not the one that bolts on the side at an angle) PM me if your interested in either of them.
I ended up with a 4wd v8 mount on the drivers side, and a 2wd v8 mount on the passenger side.
But to answer original question, i ordered mine off of ebay from a chevrolet dealer (oem mounts)