Focus test negatives

Hello,

I'm collecting test negatives used for focusing an enlarger, or testing optics. Like this one:



Does anyone have any ideas where I might be able to find these? I get them occasionally on ebay, and have found them at camera shows. I suspect for the most part they're gathering dust in the corner of old darkrooms. Maybe you've got one? Or have seen one at a bricks and mortar shop? If you speak a language other than english, perhaps you could tell me what they're called language? It would help with google searches.

Any and all ideas appreciated! Thanks!

Edmund Scientific has a good selection of chrome on glass and printed on paper resolution targets. (100-300 us$) (Science grade)
Take care that you are find analog or digital era test targets. Film does not care about rotation and line angle.
Digital imagers are not so and modern targets address that for digitals.
Adorama and B&H have one or more listed on line.