Found I have a broken spoke. I see various resources for calculating spoke length but given this is a mass-produced item, isn't there some resource to look up exactly what spokes you need? What about just measuring the existing spokes? Where is the spoke length measured from? As I understand it, the spokes can be different lengths depending on which side and whether inner/outer.

For those whose response to everything is "take it to a bike shop", save yourself the keystrokes. Presumably this is a forum for those who want to learn about bike mechanics.

Just measure the spoke end to end and buy a replacement. Get the same gauge so it matches the nipple and/or buy a matching nipple to be sure. J-bend spokes measure from the bend.
Take the spoke with you to a shop.

If it's on the drive side you'll need to remove the cassette /freewheel first.
Large cogs may also get in the way of non-drive side spokes.

It is exceptionally difficult to measure where the spokes should go in an unbuilt wheel, and thus the need to measure the rim, hub, and use spoke calculators.

With a wheel that has already been built, if you have the broken spoke at the J-bend, measure it and it should be fine.
If you have a nipple, measure length of a spoke to the nipple, then add the length of the nipple. Usually the spoke should extend all the way through the nipple.

Generally all front wheel spokes are the same length.

For rear wheel spokes, you only have a right/left difference. It doesn't matter which direction they go through the flange.

With a built wheel that's missing a spoke, it's sometimes possible to measure along an existing spoke on that wheel. Chose a spoke that is on the same side and orientation as the missing spoke. Measure from inside the bend at the hub to where the nipple enters the rim and add about 3mm. As mentioned by others, you can measure to the end of the nipple and add its length to the measurement, but you need to know the length of the nipple for that. You can sometimes do this for straight-pull spokes too if you can see where the head is, and measure from inside the head. Hidden heads, internal nipples, proprietary spokes and nipples, etc. all complicate matters.

People are making this too complicated.

You have a broken spoke. Presumably it's broken at the hub end. Unscrew it from the nipple by spinning it in your fingers, or holding it with pliers as you turn the nipple with a spoke wrench

Spokes are measured from the inside of the J-bend to the end. Your spoke broke right at the J-bend, so you need to eyeball where the inside was by what's left. Worst case you'll be off by 1mm, which is OK.

If you're not afraid of wheel truing I just pull a spoke (usually opposite) and take it with me to the shop, then buy two and replace both old ones. Worked fine the couple times I did this.

The simplest.
Ideally, you want the spoke to end at the bottom of the screw driver slot of the nipple.
Look at the other spokes on the same side of the hub and see if you want to adjust a mm or 2 to get it correct.
Many mass produced wheels aren't terribly fussy in this dimension.

Which I accept as evidence that it is not as critical as some of us OCD types, myself included, strive for.

Getting the correct length spoke depends on how accurately you can measure it. I have a spoke measure ruler floating around someplace in one of my toolboxes, but I seldom have used it. For me the best way to buy a spoke if it broke off at the j bend is to take the old one to a bike shop where they can simply line up the old with the new, nothing complicated at all. However, if I was going to order enough spokes online to build a new wheel I would haul out my trusty ruler which measures spokes from the inside of the bend. If you only need a spoke or two, go to a shop. If you need a wheels worth, measure. For me, after 45+ years of working on bikes professionally and for myself, I always choose the simplest solution. If I have to go to a shop, which is what I would do for a few spokes, I follow the line of least resistance

We're measuring by eyeballing to the vestigial J-bend, and working to 1mm accuracy. For these purposes even the cheapest painted plastic ruler like the 10 cent ones school kids carry (yes, I know 10 cent rulers aren't 10 cents anymore) are equally accurate.

IMO - people here on BF and elsewhere in the bike world tend to get worked up over a sense of precision that simply isn't relevant to the purpose.

You should be able to get a good enough estimate with the spokes in the wheel.

I'll just lay a new spoke up next to the existing spokes, and guess where the nipple will stop.

There was someone complaining about removing consecutive spokes throwing his wheel out of true. I don't know if any damage was done, but you're probably better off doing a spoke on the opposite side.

Still, that seems to be extra work.

As FBinNY mentioned, for replacing a couple of spokes, it doesn't have to be exact. If you have a single walled rim, the worst thing would be filing down a bit of spoke sticking out.

If you're building a new wheel, it is nice to start with good spokes of the right length, but the ones that came out aren't necessarily the perfect length either, especially if changing the dishing. A lot of spokes stop short of the nipple head, which can put stress on the nipple, so a perfect length will go through the nipple head.

1mm accuracy is plenty close for any spoke. I didn't mean to imply that spoke measurement was rocket science, Sorry if I gave that impression