What donor vehicle to get disc brakes setup from for 72 swinger.

Believe it or not, a kit may be your cheapest option.
Here's one based on stock parts. Your wheels will need to be 4.5" inch bolt pattern with the kit. They're probably 4" now with stock drums.

This vendor is a member of the forum and gives great customer service.
10.95" Front Disc Brake Kit (Stage 1)

Hhere's one thread (of many) that you may find helpful.
A Body Disc Brake Swap 101