The atomic bombs dropped at Alamogordo and Nagasaki were both Implosion type weapons while the Bomb dropped on Hiroshima used a gun type system.
Was this because plutonium was easier to obtain than Uranium-235? Did the United States plan on using more implosion type weapons or gun type weapons if Japan didn't surrender after the 2nd bomb?
It was about availability of fissile material, and the amount of fissile material you need in each type of bomb.
At Oak Ridge, they were producing around 1 kg of high-enriched uranium per day. But it required 64 kg of uranium for the gun-type bomb.
At Hanford, they could produce about 20 kg of plutonium a month. Each bomb required only 6.2 kg of plutonium. So at max production they could produce three bombs a month.
In the end, they had about 13 kg of plutonium and 64 kg of HEU by July/August 1945, so that means 2 plutonium bombs and one HEU bomb.
They were still producing HEU and Pu after the bombs were used. They would have had another Pu bomb in mid-August (about a week after Nagasaki), and about 3 bombs per month after that, and only later gotten another HEU bomb. Presumably if it had gone on that long they would have developed composite bombs that used both HEU and Pu (which they were already thinking about doing), in which you might have a couple kg of Pu and then maybe 10 kg of HEU, and you get about the same amount of explosive power from that as a pure Pu bomb. (This is what they did in the postwar, to make their HEU more useful, and their Pu go a longer way.)