shock you cannot see. Go on line to any number of sources from the local or national rigging place - industrial or theater, to Grainger or McMaster Carr for the proper ones. Easy enough to get and no doubt cheaper or the same price once shipping is added to the price.
Sapsis Rigging website, believe it was an early edition of "Heads" which brought this to my notice - been years since written but trust me you don't use aluminum, much less most normal crimp tools are not set up for crimping aluminum properly - different gap and gauge if memory serves. This means you might have been over-tensioning the aluminum sleeve which isn't good either. Been a while since I studied aluminum sleeves on the other hand so the different crimp tool is a question. McMaster would list the proper crimp tool for aluminum sleeves, if the same in a size as for copper, than I'm wrong but doubt I am. McMaster would also no doubt have a safety warning about aluminum sleeves.
On snap hooks (not key chain caribiners hopefully = not load rated), that's the current standard though I'm not much of a fan of them. I like Euro snap hooks with thimbles on the wire rope and a load rating tag on the safety cable much better & hope the industry goes that way soon. Unfortunately such snap hooks are not domestically available.