Hey Dahui!In the north Modoc is the place for Brahmins.In the south I always get them in the square below HUB and the square west of the one below HUB. Might take some time, since encounters have from 2 to 4 brahmins usually.
Just use FOnline Encounter Finder !
You might want to use the FOnline Encounter Finder, which you can fiund on this site (an awesome job by Opera).In the particular case of Brahmins, wandering around Modoc will give you better results.Best of luck.