Breast milk is the most natural food on Earth and — some would argue — the most essential for a baby's health. Still, it isn't something every woman can produce. For those who can't, the quest to obtain it can become a mission.

They spend hours a day finding their own donors. They surf the Internet, go to classes where new moms congregate, visit chat rooms and seek referrals from lactation consultants — all to find a nursing mother who's producing too much of a good thing and has some to spare.

Two factors fuel the trend. One, mothers have learned the many health benefits of breast milk, thanks in part to the strong encouragement from the American Academy of Pediatrics and the World Health Organization. And two, new mothers have greater access to breast milk sources through the Internet.

Breast-feeding experts agree that the find-it-yourself movement is growing. But the nature of the Internet and word-of-mouth relationships make it difficult to determine whether hundreds, or thousands, of women are seeking breast milk from unlicensed providers.

The Human Milk Banking Assn. says that the demand for human milk has increased 37% over the last four years. Last year, the association reported that its active milk banks distributed more than 560,000 ounces of milk, compared with 410,000 in 2000. During the first two months of 2005, demand has outpaced last year's by 15%, said Mary Tagge, secretary for the organization.

Although health experts don't want to malign mother's milk, the fact is, getting it from some place other than a milk bank could endanger an infant's health.

"Breast milk is a human tissue and therefore carries the same risks that transferring other human tissues carry," said Ron Harkey, section chief of tissue, blood banking and cytology surveillance for the California Department of Health Services. Babies could contract diseases such as AIDS and hepatitis, he says, or become ill from contaminated milk that hasn't been properly collected and stored.

"Your next-door neighbor might be a wonderful person," Harkey said, "but when it comes to sharing something like her milk, she could put your baby at risk."

California and New York (but no other states) even have laws that prohibit unlicensed providers from distributing breast milk, which can transmit disease.

But some women are not easily deterred. They've heard for years about the brain- and immunity-boosting benefits of breast milk, and they're determined to give their children every advantage.

Cynthia Dewey is among them. The Sherman Oaks woman and her husband adopted a newborn four months ago. She first tried, unsuccessfully, to get herself to lactate, which some women can do with enough breast stimulation either from a breast pump or a baby.

Then she considered going to a milk bank. But milk banks charge around $3 per ounce. A 3-month-old baby normally consumes 20 to 30 ounces a day. Dewey estimated she would have had to pay around $2,400 a month. "I can't afford that," she said.

Dewey never asked her doctor for a prescription — which milk banks require — because she knew she wouldn't be able to afford the milk. Besides, some doctors only write prescriptions for premature or sick babies.

"It depends on how much of an advocate the doctor is for breast milk," said Katy Lebbing, a lactation consultant and manager of the center for breast-feeding information at La Leche League International in Schaumburg, Ill.

Families who come to the milk bank typically have a child who has a medical condition that requires human milk, says Pauline Sakamoto, executive director of Mothers' Milk Bank in San Jose. The state-licensed milk bank is one of only two in California and nine in the country. "These babies are often either premature or aren't growing. Healthy adopted babies don't get priority."

As for Dewey, she fell back on a practice that's been around as long as babies have been born: Getting breast milk from other mothers.

With her pediatrician's encouragement, she went to classes for new mothers and put out an e-mail to more than 50 lactation consultants appealing for breast milk.

Eventually she cobbled together a supply for her new son. And she's never regretted it.

"Since I didn't have control of what the birth mother ate when she was pregnant with my son, I was compelled to give him the best start I could from the moment I could," she said. "Every bottle of breast milk I give him makes me feel so good when I think about all it's doing for his brain, his immune system and his heart."