In this study done in 1981 cows milk has between .0595 and .0225 grams per litre.
Commercial fruit juice has between .0187 and .1236 grams per litre.
Plain and creamed vegetables .098g and .204 grams per litre
vegetables contributed the most lead and the sources are thought to be environmental exposure - I think that means traffic / industry.
Interestingly what I found out from all this is that if you grow food in your own garden you should test for lead levels in your soil. So if you are a home grower, like I am, then get a home soil lead testing kit.