Surabhi cows are generally found on the Vaikuntha planets. As described in Brahma-samhita, Lord Krsna, on His planet, Goloka Vrndavana, engages in tending the surabhi cows (surabhir abhipalayantam [Bs. 5.29]). These cows are the Lord’s pet animals. From the surabhi cows one can take as much milk as one needs, and one may milk these cows as many times as he desires. In other words, the surabhi cow can yield milk unlimitedly. Milk is necessary for the performance of yajna. Sages know how to use milk to elevate human society to the perfection of life. Cow protection is recommended everywhere in the sastras.

From Srila Prabhupada’s purport to SB 8.8.2