Don't know if these clips are/have been available elsewhere, but.....
I got this 4-DVD collectors edition of Gone With The Wind for my husband and on one of the disks are a couple of clips from Joan Bennett's screen test. (!) It's sort of interesting. They show what appear to be the four final actresses in contention for the part, doing the same scene.
Anyway, for any Gone With The Wind fans, it's a nice release - has a lot of extras, i.e., stuff about the making of the film, a reproduction of the original booklet sold in theaters, etc. The 4-disk package is nicely put together AND it's only $28 from Deep Discount DVD. (Saw it in Best Buy for $30).