Iron Golems spawn in Villages, Pillager Outposts, and can be summoned by Villagers. They drop 3-5 iron ingots when killed, meaning you are guaranteed 3 per Iron Golem. Golems should spawn with world generation in every single village centre, with the exception of abandoned villages and 50% of Taiga villages.
Seeing a Bell means you have a village loaded, but if you need the golem, you can also check your entity pie to see if one is loaded. If you're in a Taiga and can see this Well at all (pictured to the right), then you will have a Golem: it is only the other meeting point that doesn't generate one.
It's fairly widely known you can stack up 3 blocks and crit it with an axe or sword to kill it. Be careful that there are no higher blocks nearby that the golem can get to to reach you. If it looks like they might, you can crouch on the other side of your pillar to make it pathfind beneath you instead. It takes 8 crits with a stone axe and 10 crits with a wooden axe.
The golems in Pillager Outposts spawn in the cages that can generate in the three corners around the central tower. These always have open tops, so pillaring up 3 (or double stacking and block extending) can let you drop inside, where you can mine down 3 blocks and crit out the golem from below. The cage will keep you from being annoyingly pinged by Pillagers, and you can break the logs on the corner to let yourself out.
If it is past the first sunset, Villagers can summon Iron Golems, and for Draftout this will usually be in the case of killing a Golem in a village and then summoning another one for a goal like Leash Iron Golem. See this thread by fakem0n on how to summon a new iron golem. Due to the 3 villager requirement, this won't work in Igloos.
Zombies, Husks and Zombie Villagers (note NOT Drowned) have a 0.83% chance to drop 1 iron ingot. This is equivalent to around 1 in 120.5 of these mobs, so it is not worth forcing but it is also something that you could bear in mind.
You can find Iron Ingots, Nuggets and Blocks in Loot containers in many structures. The best structures for getting a lot of Iron are (1) Ships and BTs, (2) Bastions, and (3) Trial Chambers. Below is a table with the number of Iron Ingots found on average in a given structure. (So Iron Nuggets / 9 and Iron Blocks * 9 added.) Items you can smelt into nuggets (such as Nautilus Armor) are not included in the iron count but are in the "other iron items to note" section.
Players coming from MCSR Ranked will have to adjust to Vanilla loot rates, and the biggest culprit here are Ruined Portals. In Vanilla, there is only a 7.32% chance of an RP having 3+ ingots, and a 0.002% chance of having 6+.
Trial Chamber will be missing before Sam can simulate them, but they have a lot of iron.
Anything with an asterisk in the "chests" column is neither calculated nor tested from a large sample size, but were rather made from some testing and guesswork. If anyone can provide accurate data on the number of loot chests in some of those structures it would be greatly appreciated.
Credit belongs to Sam for providing all the data used for Villages. Note that this only counts Iron Ingots, Iron Nuggets, and Iron Blocks in loot containers, so the Iron Golem is not considered. Including the Iron Golem, the average is +4 for every village type except Taiga, which is +2.
Vanilla rates are 2.13% per ingot for 10-36 iron nuggets. That means we can project the expected iron from gold blocks as follows