Where did you get your front disc conversion?

I did the old, tried and true conversion on a '68 Dart Sedan a few years back.
That conversion was as follows:

1973-'76 A body spindles.
'76-'81 B & R body 11.875 rotors.
Calipers and mounts from a '76 Cordoba (pin type).
MANUAL master cylinder from a '73 A body 6 cylinder car.

Rear brakes were true blue 11" X 3" (NOT 2.5") ones from a '67 Fury Station wagon, which, BTW, are adjusted manually ONLY.
Stopping was never an issue and the whole brake set-up, fronts and rears, cost me under $300.

Don't be scared to use the F,J or M body spindle and brake components.
They are as close to the A body ones as you'll get for a cheap price, and the difference is one bolt hole, a threaded hole versus a drilled one, for the lower ball joint.
Hardly worth hollerin' over.

Mark.