1970 Dart Swinger.

My brother wants like dual piston calipers and slotted/venter rotors, but he doesn't want to buy a WHOLE kit. He just wants the calipers and rotors.

Where should I look for some that will go with the factory spindles?

Most every place that has brake upgrades its all in a "kit." I don won no steenkin' kit!

If you don't want no steeenkin' kit, you'll have to do the steeenkin' work!
Most kits are based on stock available parts research. The only fabricated parts are usually the caliper brackets. These people who sell kits are not just selling parts, they're selling hours and hours of eyeball straining bookwork and shop work on actual vehicles refining their products for best fit and function.
If you're lucky, a later MoPar product which uses discs will share the same dimensional spindle stub as your Dart. This gives you a rotor/hub assembly to start with, hopefully using the same bearings and seals as your car. If not, get out the nearing and seaql books and see IF there are bearings and seals which will do the job.
Remember, aside from the fabricated caliper bracketry you'll want stock parts so when you have an accident or error in judgement you won't have to kill your car for weeks while you make up some custom parts (spindles/hubs etc.).

Good point. We never even considered that all the parts HAVE to work together.