Amazon has them but the reviews are overwhelmingly horrible. I have found that a short, round point shovel with a "D" shaped handle works great, I use that method on heavy motorhome tires. Fix the jack so the tire is barely 1/2" or so off the ground, put your shovel in there and lever it into place. You can twist the D handle a bit and move the shovel handle fore and aft for stud alignment. Even the little 18" shovels work.
Like with a lot of things you're going to see mixed reviews. My experience has been overwhelming positive. There are much better and more costly versions of these and they get good reviews. Personally, I'd build my own before I paid $300 (CDN) or so before I bought one though.
As for the cheap ones like mine, Some of the reviews I read said that they were too big for 20" tires or for some car tires in general. No kidding. You could tell that by looking at it so I have no idea why anyone would even consider one of these. For my 31" and 33" tires they work great. It's not something you would take on a trail with you but for around they're good for around the garage.