The federal Department of State (DOS) is the only source for a U.S. visa. No visas are issued inside the United States. Department of State Visa Services
Visit the local consular section of the U.S. missions abroad to see the latest guidance on the visa application plrocess, which varies widely from post to post: US Embassies and Consulates Worldwide
See estimates of waiting times at each post for both making an appoinment AND return of passport with visa afterwrds: Visa Wait Times
DOS manages the allocation of Immigrant Visas on a monthly basis througout the fiscal year [1 October - 30 September].,
Priority Dates for Immigrant Visas, both Employment and Family, by Category and Country are found in the monthly Visa Bulletin
U.S. Passports are available to U.S. citizens on an expedited basis [1 -2 days] for those with clear proof of citizenship and an immediate need to travel: US Passport Services
DOS manages the annual raffle of 50,000 Immigrant Visas via the "Green Card Lottery"