Wel, it comes in bigger bottles around here, in Latvia, and you can buy it even in bigger grocery stores... I like buying it in 500 ml bottles, so that I can use it as fresh as possible, and vitamin A is not the only beneficial component of it.
I didn't manage to find a precise source on the amount of vitamin A in milk thistle oil, but this is a good source of general information of its' benefits - http://www.nutraceutical.com/educate...lk_thistle.pdf
And this contains some good information about the nutrients, too - http://www.nifa.org.pk/FSDMilkThistle.htm
In our stable, we have used it in feeding horses ho have had rainscald, and a horse with weak livers - their health noticeably improved and rainscald isn't an issue anymore, although they are all pastured 24/7, the climate is moist and rather warm, and none of the horses are clipped.