I don't know if you can buy the part in question but it is pretty easy to disassemble and I don't think it is going to void the warrenty. I've disassembled the lever a few times to clean out dirt.
If you remove the screw (top) that holds in the lever, a small rounded wedge (bottom) will come out. You may have to thump the ASA against your palm to get it out. The .5" spring should be in that wedge piece. Just remember how it goes back into the ASA. The spring should be pointing up towards the bolt and the spring side (versus the slanted side) should go towards the front grip.