The majority of people get more than adequate amounts of the amino acid glutamine in their diets. High quality sources are lean protein (beef, pork, fish, poultry), eggs, cottage cheest, milk, yogurt, dried lentils, beans and peas, raw green vegetables (spinach, endive), and other vegetables like kale, beets,Brussels sprouts, carrots, etc. Most people have intake of 1 to 6 grams per day.
L-glutamine is available as a supplement, or as part of a protein additive. Normal intake in 500 mg per day for people who have cancer, HIV, inflammatory bowel disease or are suffering from extreme injury or wound healing.
Here is a chart showing how much is in a few common foods. You can ignore everything else on the page, except the chart, since it's just an advertisement.