After searching and pricing the cables for my 1985 last year I decided that purchasing the pre-fab cables from J&R was easily the most economical but also the safest and easiest way to go.
The threaded piece is obtainable from any good cable seller. Now you will need a good Swag tool, $100-500+. Then all of the fittings, thimbles and swags/collars, and cable cutter,($50/90). The tools at HD and Lowes are not the quality or correct sizes, I know because I tried that route first also.
Once you add all of the parts and tools, the cost is still not the most important piece of the pie. Swagging cable is not hard, but doing it correctly is a matter of safety above all else.
If you don't have this skill set it is best to let the experts do it. I have for many years swagged cables for hang gliders/aviation and know what I am doing. The safely level is serious at this level since death can be a result of improperly swagged cables.
If one of your cables fails it can lead to other cables being over stressed and cascading failure, resulting in trashing your trlr frame structure, and possibly injuring a family member.
This can be a DIY project, but even with my skill set, I preferred to purchase the correct length. The $ cost to do it correctly is better used on a non safety project in my experience and personal call.
If you decide to DIY buy very good tools and materials and practice. I am a DIY kind of guy and this is my personal and professional opinion.
There are places to DIY and I believe strongly this isn't the place or part.
Not trying to rain on your parade!
steve