Discounts for cash. Cosigning a loan. Electronic banking. Life insurance and savings. Buying diamonds. Protein for athletes. Loose-fill home insulation.

Hundreds of publications on those and other such subjects are available, most of them free, from local and federal agencies trying to provide consumers with information that can help them avoid costly buying mistakes.

The vast government library of consumer material ranges from one-page bulletins on specific topics, such as "Door to Door Sales" from the Federal Trade Commission, to hefty magazine treatises, such as "Pregnancy and Drugs" from the Food and Drug Administration.

Some wall posters suitable for classroom use also are available. One from the FTC, for example, shows how homeowners can use R-Value numbers to choose the proper insulation.

But because of government publication policy and spending cuts, it's uncertain how much longer consumers will be able to get these publications.

Departments and agencies must seek approval by next Jan. 15 for each magazine and pamphlet they want to continue publishing, according to a directive issued this week by Office of Management and Budget Director David A. Stockman. In addition, the departments and agencies have been told to develop a publication plan for periodicals.

Then there are the budget cuts that affect essentially all government operations, including the giveaway consumer brochures.

The FTC's Office of Consumer Education, the section in charge of publications, had a budget of $348,000 during fiscal 1981. That is $8,000 more than the $340,000 budget proposed for fiscal '82. And now that the FTC must cut expenses by 12 percent to meet President Reagan's new budget goals, the $340,000 budget for publications may have to be reduced even more.

"We don't know yet what the budget for publications will be," said FTC representative Neil Friedman. "But we might have to go from glossy stock paper on some publications to something less elaborate."

Many other government agencies also face cutbacks. Meantime, here are examples of some of the free materials still available:

* Gasoline octane ratings, eyeglasses, credit, insurance, diamonds, cash discounts, insulation and banking are among the 40 to 50 subjects covered by various FTC publications. The office also has a "best-seller" list of its most popular publications. To obtain a copy of the list or of a publication on a particular subject, write the FTC's Office of Consumer Education, Sixth Street and Pennsylvania Avenue NW, Washington, D.C., 20580. The telephone number for the consumer office is 724-1870. Consumers need only state the topic they want; the FTC will provide what it has available on that topic.

* Food additives, labeling, pregnancy and drugs, radiation and cosmetics are among the topics in the Food and Drug Administration reservoir of publications. Consumers should state the topic they want on a postcard rather than a letter and mail it to the Food and Drug Administration Office of Consumer Affairs, Consumer Communications Staff, HFE-88, 5600 Fishers Lane, Rockville, Md., 20857. Or call the office at 443-3170.

* Nutrition, food preservation, gardening and farming are among the topics featured in publications from the U. S. Agriculture Department. To obtain a copy of any of them, however, the consumer will need a specific title or publication number. The list of titles and numbers can be ordered from the USDA Information Office, Washington, D.C., 20250. In addition, consumers can ask at their public library for the list of available USDA publications. The telephone number for information about the USDA publications is 447-2791.

Federal publications also can be ordered from a catalogue available from the Consumer Information Center, Pueblo, Colo., 81009. Many of them also are often available from local consumer agencies -- along with pamphlets and brochures written by the local office for its constituency.

The Fairfax County Office of Consumer Affairs, for example, has put out a comprehensive handbook for tenants in Fairfax County. Montgomery County compiled a guide to nursing homes in Montgomery County. And Alexandria's Office of Consumer Affairs has a booklet on its local home improvement code.

Local offices with publications include:

* Alexandria Office of Consumer Affairs, City Hall Box 178, Alexandria, Va., 22313. Telephone 838-4350.

* Arlington Office of Consumer Affairs, 2049 N. 15th St., Arlington, Va., 22201. Telephone 558-2142.

* District of Columbia Office of Consumer Protection, 1424 K St. NW, Second Floor, Washington, D.C., 20005. Telephone 727-1294.

* Fairfax County Office of Consumer Affairs, 4031 University Dr., Fairfax, Va., 22030. Telephone 691-3214.

* Montgomery County Consumer Office, 611 Rockville Pike, Room 201, Rockville, Md., 20852. Enclose a stamped, self-addressed business-size envelope when ordering a publication. Telephone 279-1776.

* Prince George's Consumer Protection Commission, County Administration Building, Upper Marlboro, Md., 20772. Telephone 952-4700.