Sperm banks. Commercial sperm banks. A simple internet search will yield several results. These banks will provide information on the details of the donor. Once ready for treatment, your doctor may need to fill out a form for obtaining the sperm.
Known vs. anonymous. Donors fall under two classifications: known or anonymous. Most physicians use mainly anonymous donors; there are agencies nationwide easily found on the internet. Known donors can be friends, relatives, or otherwise, but regulations for screening are strict so many professionals prefer not to use them. State laws regarding paternity with known donors are also variable, so legal advice is a must.