A family member is tasked with getting 3-pin DMX cables for their rotary group, and I've soldered hundreds of mic cables in my lifetime, so I volunteered to help make them at the lengths they need. It's likely they'll need about 1,000' of cables, possibly twice this. Where should I look for bulk cable, and what kind should I get? This will be for a lighting rig that moves around, not permanent instillation, so they don't want to go with the cheapest option, they want to make sure it will last. That being said, they also only do 5-10 shows a year, so they aren't going to be in use every day. Any suggestions for the cable to use, and where to get it, would be appreciated. I'll be soldering on the ends myself, so I'm looking for just the bulk cable, not premade ones. Thanks!

I'm sure some Belden cable would work well but you didn't say where you are so I can't help with finding it.

What you might consider is making at least some of the DMX system wireless, if nothing else do it with the long run back to the controller. If the fixtures are arranged in groups on separate stands wireless also accomplishes signal splitting so no need to have a continuous cable run to all fixtures. If they are dead set against using wireless but do have a need to feed the signal in 2 or more directions then buy a hardware splitter so you don't have to run cable all the way to one side and then all the way back to the other, that should save some cable and reduce setup times.

As I understand it, they are running some things wireless, but a lot of times this is being setup and run by people who are not lighting professionals (or even hobbyists). They have a system that is working fairly well right now, and they just want to replace their old crappy mic cables with better quality cabling specifically for the DMX. I don't think there is any interest in upgrading or modifying how they do things, so it's going to be a lot of cables, both short and long, to get things hooked up.

I'm in the USA, near Seattle WA.

How many fixtures are in the lighting rig?
Are they experiencing problems of any type?

Depending on the number of fixtures and the distances covered the cable itself may or may not be significant, but that doesn't account for wear and tear on the cables and connectors... they may be due for replacement anyway. Do they have DMX terminators at the ends of cable runs now? These should always be used, if they experience uncontrolled glitches of any type and they aren't using terminators then simply adding them will usually solve that problem. Here are a couple links I found to bulk cable..
https://www.fullcompass.com/prod/519...lighting-cable
https://www.fullcompass.com/prod/585...4awg-dmx-cable

I already told them about the terminators, but I will bring that up again. I appreciate the help and advice!

Monoprice is my go-to for bulk cable, connectors as well as pre-made lengths.