I think most people are making their own. I've seen some people use grilling baskets, and while I've never tried it, I'd have to think they wouldn't last long given the high temperatures of the firebox.
Expanded metal is the material of choice for the better one's I've seen. You'll have to do a search to find it, but the best one I've seen, the basket hangs on the grate holders in the firebox. This keeps the bottom of the basket above the ash tray, allowing you to remove the tray without disrupting the basket.