Hi Alastair,
I'm in agreement that it's not possible to find better quality than Viz people - I have used MX greenscreen on occasion - they are mainly video, but stills with alpha can also be bought, they are consistent with lighting color balance etc. but are a bit corporate in feel If anyone knows of an alternative viz people with more character variety I'd be thrilled to know! Nothing looks worse than loads of different lit, inappropriate people in a scene.
