Lilly Neubauer shops Costco for paper towels, lentils and carrots — and now, even a home mortgage.

While Ms. Neubauer, 27, said she was surprised to find the warehouse club selling financial products, she and her husband saved about $200 a month by refinancing through its Web site this year. She also bought home insurance from Costco, she said, again because it was cheaper there.

“It opened us up to the fact that Costco is more than toilet paper,” said Ms. Neubauer, who lives in Dallas.

As the nation’s largest banks stay stingy with credit and a growing portion of the population has no bank at all, major retailers are stepping into the void. Customers can now withdraw cash at an A.T.M. with a prepaid card from Walmart, take out a loan at Home Depot for a kitchen renovation or kick-start a new venture with a small-business loan from Sam’s Club. This year, Walmart even started to test selling a life insurance policy.

Consumer advocates are torn about the growth of this shadow banking industry. Financial products are making it into the hands of people who otherwise might not qualify for them, but these products are not always subject to the same regulations as bank products are. And to turn a profit, retailers generally have to charge more to people with poor credit or none at all.