I was wondering if there is a car or truck with disc brakes that would fit the spindles on my 56 f100? Don't want to buy a kit from aftermarket if I can help it.

 

In general, at least in my experience, the kits are just that - standard calipers and rotors (could be from truck or car)
The main added value of the kits are the brackets they offer to attach and properly position the calipers (plus some add-ins like mounting hardware, brake lines and tabs).
Example with a good photo/drawing of a bracket):
https://www.ford-trucks.com/forums/1...ent-issue.html

A search here on the forum gives various calipers etc that people have used and made their own brackets. It seems pretty straight forward.
I don't have any specific recommendations for you (was looking for '49 F3 options), maybe others can give options known to work.
Here is some info:
https://www.ford-trucks.com/forums/6...sc-brakes.html

I did comparisons for a couple of "turn-key" disc brake kits vs. trying to buy components on my own (and fabbing the rest); the kits were maybe $50 more than buying/making things separately.
Yes, going to a junkyard to find calipers etc is likely the cheaper way, but - at least for me - was not worth it (and I did get my engine and driveline from the junkyard)

Some things to consider:
- you may need a new master/cylinder/proportioning valve going from drum to disc
Background: https://www.motor.com/magazine-summa...nder-selection

- fitment of the disc brake setup with the wheels you have; may need new (larger)wheels.
Example: https://www.ford-trucks.com/forums/1...brake-kit.html
Wheel offset also needs to be considered.